Add bring-your-own-buffer versions of os.lookup_env and os.get_env

And make `core:terminal` use it so that `core:log` can be imported with `-default-to-nil-allocator`,
in which the actual allocator is set up in `main()`.

Windows was tricky because of the utf-8 <> utf-16 conversion, so we use some temporary stack buffers for that purpose,
limiting the non-allocating version there to 512 utf-16 characters each for the key and environment value.

In general the value is (obviously) limited to the size of the supplied buffer, and a `.Buffer_Full` error is returned
if that buffer is insufficient. If the key is not found, the procedure returns `.Env_Var_Not_Found`.

TODO:
- Factor out buffer-backed utf8 + utf16 conversion to `core:sys/util` to more easily apply this pattern.
- Add similar `lookup_env` and `get_env` procedures to `core:os/os2`.

Fixes #5336

core/os/errors.odin

@@ -35,6 +35,9 @@ General_Error :: enum u32 {
 
 	File_Is_Pipe,
 	Not_Dir,
+
+	// Environment variable not found.
+	Env_Var_Not_Found,
 }
 
 
@@ -82,6 +85,7 @@ error_string :: proc "contextless" (ferr: Error) -> string {
 		case .Pattern_Has_Separator: return "pattern has separator"
 		case .File_Is_Pipe:      return "file is pipe"
 		case .Not_Dir:           return "file is not directory"
+		case .Env_Var_Not_Found: return "environment variable not found"
 		}
 	case io.Error:
 		switch e {