ci: Stable libomp install

[j-s-ashley]

Resolves #19 , addresses the following items from #10 > build-wheels.yml > - [ ] Remove test jobs > - [ ] Use GHA cibuildwheel instead of build-wheel script > - [ ] Use GHA pypa/publish

For MacOS builds (currently just MacOS-13), this will:

1. Use the GH checkout action to access the entire LLVM project
2. Build and install OpenMP with appropriate compiler flags

[j-s-ashley]

This needs:

• [x] Commit cleanup from a partially-bungled rebase (some commits don't do what they say they do :clown_face:)
• [x] Conflict resolution
• [x] Remove snprintf commit, since that's a separate PR
• [x] Remove commits pertaining to #10