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.
Oops. The issue here isn't the name of the local. It's that I shouldn't have made the change to this line since `otherSet` is already a strongly-typed `HashSet<T>`