* Replace the separate Prefix and Mask Int fields in the Bin constructor
  with a single Int field which contains both merged together. This
  reduces the memory required by a Bin from 5 to 4 words, at the cost of
  more computations (which are cheap bitwise ops) being necessary for
  certains operations. This follows a similar change done for IntMap.Bin.

* Benchmarks show that runtimes for most operations remain unchanged or
  decrease by a small amount (<10%). As expected, allocations are
  consistently lower by 11-16% for all set operations that have to
  make O(log n) allocations.

* The functions and types used by both IntSet and IntMap have been moved
  into a IntTreeCommons module.