crypto: Add rand_bytes

This adds `rand_bytes(dst: []byte)` which fills the destination buffer
with entropy from the cryptographic random number generator.  This takes
the "simple is best" approach and just directly returns the OS CSPRNG
output instead of doing anything fancy (a la OpenBSD's arc4random).

core/crypto/rand_linux.odin

@@ -0,0 +1,37 @@
+package crypto
+
+import "core:fmt"
+import "core:os"
+import "core:sys/unix"
+
+_MAX_PER_CALL_BYTES :: 33554431 // 2^25 - 1
+
+_rand_bytes :: proc (dst: []byte) {
+	dst := dst
+	l := len(dst)
+
+	for l > 0 {
+		to_read := min(l, _MAX_PER_CALL_BYTES)
+		ret := unix.sys_getrandom(raw_data(dst), to_read, 0)
+		if ret < 0 {
+			switch os.Errno(-ret) {
+			case os.EINTR:
+				// Call interupted by a signal handler, just retry the
+				// request.
+				continue
+			case os.ENOSYS:
+				// The kernel is apparently prehistoric (< 3.17 circa 2014)
+				// and does not support getrandom.
+				panic("crypto: getrandom not available in kernel")
+			case:
+				// All other failures are things that should NEVER happen
+				// unless the kernel interface changes (ie: the Linux
+				// developers break userland).
+				panic(fmt.tprintf("crypto: getrandom failed: %d", ret))
+			}
+		}
+
+		l -= ret
+		dst = dst[ret:]
+	}
+}