Avoid boxing ImmutableHashSet<T>.Enumerator

This introduces a pattern that can be applied anywhere to conveniently avoid boxing one's own Enumerator struct without duplicating the body of foreach loops everywhere.

Fixes #211 

If folks like this, I can add a commit that applies the pattern to all the other immutable collection types.
readonly? (for enumeratorObject, definitely not enumeratorStruct... in fact ensuring that doesn't become readonly in the future might warrant a comment)