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Our implementation doesn't use Win32 CRITICAL_SECTIONs; primarily because this would involve exposing <windows.h> to all people who include <threads.h>, or duplicating the definition of CRITICAL_SECTION (messy). Instead, we implement a locking primitive loosely inspired by Linux' futexes. Ignoring support for recursive mutexes, our mutex implementation works as follows:on top of Windows' event handles

1. Every mutex has two members, _WaitEvHandle, which is a automatic reset Win32 event handle used for blocking, and _State, which is a 32-bit integerThreadId, the ID of the current thread
2. _State is initialized to -10
3. To lock, mtx_lock calls InterlockedIncrement on _State. If the return value is 0, we successfully locked the mutex. If it is not 0, then we must block, so go ahead and block on the event handle
4. To unlock, mtx_unlock calls InterlockedDecrement on _State. If the return value is -1, nobody was waiting and we can return without touching the kernel; otherwise, we must SetEvent the event handle in order to unblock a thread
5. Note that at no point once someone has blocked on the mutex will the _State member return to -1 until all threads blocked on the mutex 

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