Refactoring and improved test coverage.
Simplify package structure for Set.

This commit removes debox.set package, move Set to debox. In the
long run we may want to use a name other than Set, to avoid
conflicting with Scala's Set. For now it works though.
Basic Map clean up.

This simplifies the Map code a bit and fixes a few bugs.
We need to add some real tests now.
Add Map testing, more Map methods, etc.

This commit removes a bunch more cruft, and gets Map reliably
passing some real property-based tests. There is still a lot
more test coverage to be had, but this at least assures that
the basic update/remove/apply/contains are working correctly.

The commit also adds equals/hashCode and some other things.

Still to be done:

  * Needs a shrink() method.
  * Both Set and Map need toStandard and toIterable methods.
  * More testing.
  * Additional benchmarking on Map and Set.