Add Semaphore support

[sheaf]

Description

This PR adds support for Windows Semaphores. I mostly followed the information on this page and its linked pages.

This is my first time contributing to this library, so there might be some obvious oversights. Thanks.

Motivation and Context

This adds a counterpart to the posix semaphore functionality offered by the unix package, which I am interested in using in ghc to improve contention with package-level parallelism.

Types of changes

• [x] New feature

Checklist:

• [x] My code follows the code style of this project.
• [x] I have updated the documentation accordingly.
• [x] I have not added a new Haskell dependency.
• [x] I have included a changelog entry.
• [x] I have not modified the version of the package in Win32.cabal.
• [x] I have added tests for the new feature.

[Mistuke]

Awesome! This is looking great, let me know whenever you're ready for a review! Thanks!

[sheaf]

Awesome! This is looking great, let me know whenever you're ready for a review! Thanks!

Thanks, I think this is ready for review, now that I've got a test checking the functionality.

[Mistuke]

Thanks this looks perfect.