Summary:
In anticipation of using this to represent sets of CacheEntryRole for including or excluding kinds of blocks in block cache tiers, add significant new features to SmallEnumSet, including at least:
* List initialization
* Applicative constexpr operations
* copy/move/equality ops
* begin/end/const_iterator for iteration
* Better comments
Pull Request resolved: https://github.com/facebook/rocksdb/pull/11178
Test Plan: unit tests added/expanded
Reviewed By: ltamasi
Differential Revision: D42973723
Pulled By: pdillinger
fbshipit-source-id: 40783486feda931c3f7c6fcc9a300acd6a4b0a0a
