Enable optimizations
Make nullToEmpty @inlined
Rename and reorder local variables to be consistent
Changed iteration in union to be more efficient.

This is very similar to the approach from HashMap.merged, but we do not
call Integer.bitCount and thus loop until both masks are zero.
Optimized the loop in intersect0 to use the new bitmap-based approach
Used new loop in diff0 as well
Narrowed down loop exit criteria and added some comments
Re-enabled other benchmarks
Merge branch 'master' of https://github.com/rklaehn/hashset

Conflicts:
	src/main/scala/scala/collection/immutable2/HashSet.scala