Refactor Buffer, fix some more bugs, add more tests.

Now we're starting to get somewhere! This commit removes the
idiosyncratic Mutable/Immutable classes and replaces them
with a single, mutable Buffer. Seriously, if you want immutable
types, Debox is not (yet) the project for you. Maybe we'll
eventually get there, but for now we need to take baby steps.
Remove Vector, and extraneous tests and benchmarks.

This Vector version was never unequivically better than Scala's,
so we should remove it until we figure that out. Also, as said
in the previous commit, Debox is focusing on mutable, array-based
structures first.
Move Unit1 and Unit2 to Util.scala.