Support deriving CLike

[LunaBorowska]

Fixes #22. The custom derive code is based on enum-map crate.

Some notes:

• It's possible to make enum-set re-export derive macro provided by enum-set-derive, but it is a bit hacky.
• This needs to be a separate crate due to how procedural derive macros work.
• It's possible to remove 32 variants limit, however it likely involves backward compatibility break.

[FlashCat]

Thanks for the pull request, and welcome! The contain-rs team is excited to review your changes, and you should hear from @reem (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

[bowbahdoe]

Any updates on this PR? I don't think this will require a separate crate anymore