C++20 feature: counting_semaphore and binary_semaphore

[nmcclatchey]

Issue tracking implementation of counting_semaphores, introduced in C++20.

See counting_semaphore and binary_semaphore on cppreference.com.

Potential implementations:

• Semaphore objects in Windows XP and above.
• WaitOnAddress in Windows 8 and above. Note: Ensuring that Windows has access to the correct address will require using Windows' Compare-Exchange functions on a regular integer, rather than the standard C++ atomics.