Drop old semaphore implementation

- Removed platform-specific implementations.
- Now all semaphores are in-object, unless they need to be conditionally created.
- Similarly to `Mutex`, provided a dummy implementation for when `NO_THREADS` is defined.
- Similarly to `Mutex`, methods are made `const` for easy use in such contexts.
- Language bindings updated: `wait()` and `post()` are now `void`.
- Language bindings updated: `try_wait()` added.

Bonus:
- Rewritten the `#ifdef` in `mutex.h` to meet the code style.

core/os/mutex.h

@@ -34,7 +34,7 @@
 #include "core/error_list.h"
 #include "core/typedefs.h"
 
-#if !(defined NO_THREADS)
+#if !defined(NO_THREADS)
 
 #include <mutex>