Set operations are indepenednt on implementation.

However, when our in case of our implementation , optimalizations are used.
Map union/intersection no longer needs PersistentMapImpl
Map union/intersection tests
Merge remote-tracking branch 'origin/master' into fix_union_intersect

Conflicts:
	lib/src/set.dart
	lib/src/set_impl.dart
Merge pull request #38 from vacuumlabs/fix_union_intersect

Fixed union/intersect implementation depenedency