Add an enum_set! macro

[linkmauve]

This is a convenience macro creating an EnumSet from a list of items, pretty similar to vec! for Vec.

[FlashCat]

Thanks for the pull request, and welcome! The contain-rs team is excited to review your changes, and you should hear from @gankro (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 The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.