More deprecations, literal helpers

[mlms13]

This deprecates the remaining functions that were mentioned in the last release notes. Additionally, it adds:

• literalBool, literalTrue, and literalFalse
• intUnion (like stringUnion, replacing variantFromInt)
• union (for building custom unions that follow the intUnion and stringUnion pattern)

[mlms13]

Unfortunately, I turned off Circle CI. The v2 branch is building with Github Actions, but unfortunately this is targeting main which isn't set up to do a BuckleScript build in Github Actions. So you'll just have to trust me that this builds and I added all the right tests and they all pass.