Implement core:thread and core:sync on Unix using pthreads

Also do some cleanup and refactoring of the thread, sync and time APIs.

- remove 'semaphore_release' because 'post' and 'wait' is easier to understand

- change 'semaphore_wait' to '*_wait_for' to match Condition

- pthreads can be given a stack, but doing so requires the user to set up the guard
  pages manually. BE WARNED. The alignment requirements of the stack are also
  platform-dependant; it may need to be page size aligned on some systems.
  Unclear which systems, however. See 'os.get_page_size', and 'mem.make_aligned'.
  HOWEVER: I was unable to get custom stacks with guard pages working reliably,
  so while you can do it, the API does not support it.

- add 'os.get_page_size', 'mem.make_aligned', and 'mem.new_aligned'.

- removed thread return values because windows and linux are not consistent; windows returns 'i32'
  and pthreads return 'void*'; besides which, if you really wanted to communicate how the
  thread exited, you probably wouldn't do it with the thread's exit code.

- fixed 'thread.is_done' on Windows; it didn't report true immediately after calling 'thread.join'.

- moved time related stuff out of 'core:os' to 'core:time'.

- add 'mem.align_backward'

- fixed default allocator alignment
  The heap on Windows, and calloc on Linux, both have no facility to request alignment.
  It's a bit of hack, but the heap_allocator now overallocates; `size + alignment` bytes,
  and aligns things to at least 2.
  It does both of these things to ensure that there is at least two bytes before the payload,
  which it uses to store how much padding it needed to insert in order to fulfil the alignment
  requested.

- make conditions more sane by matching the Windows behaviour.
  The fact that they were signalled now lingers until a thread tries to wait,
  causing them to just pass by uninterrupted, without sleeping or locking the
  underlying mutex, as it would otherwise need to do.
  This means that a thread no longer has to be waiting in order to be signalled, which
  avoids timing bugs that causes deadlocks that are hard to debug and fix.
  See the comment on the `sync.Condition.flag` field.

- add thread priority: `thread.create(worker_proc, .High)`

core/os/os_darwin.odin

@@ -14,6 +14,142 @@ Errno     :: distinct int;
 
 INVALID_HANDLE :: ~Handle(0);
 
+ERROR_NONE: Errno : 0;
+EPERM:		Errno : 1;		/* Operation not permitted */
+ENOENT:		Errno : 2;		/* No such file or directory */
+ESRCH:		Errno : 3;		/* No such process */
+EINTR:		Errno : 4;		/* Interrupted system call */
+EIO:		Errno : 5;		/* Input/output error */
+ENXIO:		Errno : 6;		/* Device not configured */
+E2BIG:		Errno : 7;		/* Argument list too long */
+ENOEXEC:	Errno : 8;		/* Exec format error */
+EBADF:		Errno : 9;		/* Bad file descriptor */
+ECHILD:		Errno : 10;		/* No child processes */
+EDEADLK:	Errno : 11;		/* Resource deadlock avoided */
+ENOMEM:		Errno : 12;		/* Cannot allocate memory */
+EACCES:		Errno : 13;		/* Permission denied */
+EFAULT:		Errno : 14;		/* Bad address */
+ENOTBLK:	Errno : 15;		/* Block device required */
+EBUSY:		Errno : 16;		/* Device / Resource busy */
+EEXIST:		Errno : 17;		/* File exists */
+EXDEV:		Errno : 18;		/* Cross-device link */
+ENODEV:		Errno : 19;		/* Operation not supported by device */
+ENOTDIR:	Errno : 20;		/* Not a directory */
+EISDIR:		Errno : 21;		/* Is a directory */
+EINVAL:		Errno : 22;		/* Invalid argument */
+ENFILE:		Errno : 23;		/* Too many open files in system */
+EMFILE:		Errno : 24;		/* Too many open files */
+ENOTTY:		Errno : 25;		/* Inappropriate ioctl for device */
+ETXTBSY:	Errno : 26;		/* Text file busy */
+EFBIG:		Errno : 27;		/* File too large */
+ENOSPC:		Errno : 28;		/* No space left on device */
+ESPIPE:		Errno : 29;		/* Illegal seek */
+EROFS:		Errno : 30;		/* Read-only file system */
+EMLINK:		Errno : 31;		/* Too many links */
+EPIPE:		Errno : 32;		/* Broken pipe */
+
+/* math software */
+EDOM:		Errno : 33;		/* Numerical argument out of domain */
+ERANGE:		Errno : 34;		/* Result too large */
+
+/* non-blocking and interrupt i/o */
+EAGAIN:			Errno : 35;		/* Resource temporarily unavailable */
+EWOULDBLOCK: 	Errno : EAGAIN;		/* Operation would block */
+EINPROGRESS: 	Errno : 36;		/* Operation now in progress */
+EALREADY:		Errno : 37;		/* Operation already in progress */
+
+/* ipc/network software -- argument errors */
+ENOTSOCK:			Errno : 38;		/* Socket operation on non-socket */
+EDESTADDRREQ:		Errno : 39;		/* Destination address required */
+EMSGSIZE:			Errno : 40;		/* Message too long */
+EPROTOTYPE:			Errno : 41;		/* Protocol wrong type for socket */
+ENOPROTOOPT:		Errno : 42;		/* Protocol not available */
+EPROTONOSUPPORT:	Errno : 43;		/* Protocol not supported */
+ESOCKTNOSUPPORT:	Errno : 44;		/* Socket type not supported */
+ENOTSUP:			Errno : 45;		/* Operation not supported */
+EPFNOSUPPORT:		Errno : 46;		/* Protocol family not supported */
+EAFNOSUPPORT:		Errno : 47;		/* Address family not supported by protocol family */
+EADDRINUSE:			Errno : 48;		/* Address already in use */
+EADDRNOTAVAIL:		Errno : 49;		/* Can't assign requested address */
+
+/* ipc/network software -- operational errors */
+ENETDOWN:		Errno : 50;		/* Network is down */
+ENETUNREACH:	Errno : 51;		/* Network is unreachable */
+ENETRESET:		Errno : 52;		/* Network dropped connection on reset */
+ECONNABORTED:	Errno : 53;		/* Software caused connection abort */
+ECONNRESET:		Errno : 54;		/* Connection reset by peer */
+ENOBUFS:		Errno : 55;		/* No buffer space available */
+EISCONN:		Errno : 56;		/* Socket is already connected */
+ENOTCONN:		Errno : 57;		/* Socket is not connected */
+ESHUTDOWN:		Errno : 58;		/* Can't send after socket shutdown */
+ETOOMANYREFS:	Errno : 59;		/* Too many references: can't splice */
+ETIMEDOUT:		Errno : 60;		/* Operation timed out */
+ECONNREFUSED:	Errno : 61;		/* Connection refused */
+
+ELOOP:			Errno : 62;		/* Too many levels of symbolic links */
+ENAMETOOLONG:	Errno : 63;		/* File name too long */
+
+/* should be rearranged */
+EHOSTDOWN:		Errno : 64;		/* Host is down */
+EHOSTUNREACH:	Errno : 65;		/* No route to host */
+ENOTEMPTY:		Errno : 66;		/* Directory not empty */
+
+/* quotas & mush */
+EPROCLIM:		Errno : 67;		/* Too many processes */
+EUSERS:			Errno : 68;		/* Too many users */
+EDQUOT:			Errno : 69;		/* Disc quota exceeded */
+
+/* Network File System */
+ESTALE:			Errno : 70;		/* Stale NFS file handle */
+EREMOTE:		Errno : 71;		/* Too many levels of remote in path */
+EBADRPC:		Errno : 72;		/* RPC struct is bad */
+ERPCMISMATCH:	Errno : 73;		/* RPC version wrong */
+EPROGUNAVAIL:	Errno : 74;		/* RPC prog. not avail */
+EPROGMISMATCH:	Errno : 75;		/* Program version wrong */
+EPROCUNAVAIL:	Errno : 76;		/* Bad procedure for program */
+
+ENOLCK:	Errno : 77;		/* No locks available */
+ENOSYS:	Errno : 78;		/* Function not implemented */
+
+EFTYPE:	Errno : 79;		/* Inappropriate file type or format */
+EAUTH:	Errno : 80;		/* Authentication error */
+ENEEDAUTH:	Errno : 81;		/* Need authenticator */
+
+/* Intelligent device errors */
+EPWROFF:	Errno : 82;	/* Device power is off */
+EDEVERR:	Errno : 83;	/* Device error, e.g. paper out */
+EOVERFLOW:	Errno : 84;		/* Value too large to be stored in data type */
+
+/* Program loading errors */
+EBADEXEC:	Errno : 85;	/* Bad executable */
+EBADARCH:	Errno : 86;	/* Bad CPU type in executable */
+ESHLIBVERS:	Errno : 87;	/* Shared library version mismatch */
+EBADMACHO:	Errno : 88;	/* Malformed Macho file */
+
+ECANCELED:	Errno : 89;		/* Operation canceled */
+
+EIDRM:		Errno : 90;		/* Identifier removed */
+ENOMSG:		Errno : 91;		/* No message of desired type */
+EILSEQ:		Errno : 92;		/* Illegal byte sequence */
+ENOATTR:	Errno : 93;		/* Attribute not found */
+
+EBADMSG:	Errno : 94;		/* Bad message */
+EMULTIHOP:	Errno : 95;		/* Reserved */
+ENODATA:	Errno : 96;		/* No message available on STREAM */
+ENOLINK:	Errno : 97;		/* Reserved */
+ENOSR:		Errno : 98;		/* No STREAM resources */
+ENOSTR:		Errno : 99;		/* Not a STREAM */
+EPROTO:		Errno : 100;		/* Protocol error */
+ETIME:		Errno : 101;		/* STREAM ioctl timeout */
+
+ENOPOLICY:	Errno : 103;		/* No such policy registered */
+
+ENOTRECOVERABLE:	Errno : 104;		/* State not recoverable */
+EOWNERDEAD:			Errno : 105;		/* Previous owner died */
+
+EQFULL:	Errno : 106;		/* Interface output queue is full */
+ELAST:	Errno : 106;		/* Must be equal largest errno */
+
 O_RDONLY   :: 0x00000;
 O_WRONLY   :: 0x00001;
 O_RDWR     :: 0x00002;
@@ -133,6 +269,7 @@ foreign libc {
 	@(link_name="write")   _unix_write   :: proc(handle: Handle, buffer: rawptr, count: int) -> int ---;
 	@(link_name="lseek")   _unix_lseek   :: proc(fs: Handle, offset: int, whence: int) -> int ---;
 	@(link_name="gettid")  _unix_gettid  :: proc() -> u64 ---;
+	@(link_name="getpagesize") _unix_getpagesize :: proc() -> i32 ---;
 	@(link_name="stat")    _unix_stat    :: proc(path: cstring, stat: ^Stat) -> int ---;
 	@(link_name="access")  _unix_access  :: proc(path: cstring, mask: int) -> int ---;
 
@@ -287,6 +424,16 @@ dlerror :: proc() -> string {
 	return string(_unix_dlerror());
 }
 
+get_page_size :: proc() -> int {
+	// NOTE(tetra): The page size never changes, so why do anything complicated
+	// if we don't have to.
+	@static page_size := -1;
+	if page_size != -1 do return page_size;
+
+	page_size = int(_unix_getpagesize());
+	return page_size;
+}
+
 
 _alloc_command_line_arguments :: proc() -> []string {
 	res := make([]string, len(runtime.args__));