OpenHashSet fixes

Incorporated ideas from pull request #200.
- Use Murmur Hash 3 finalization step to scramble the bits of HashCode
  instead of the simpler version in java.util.HashMap; the latter one
  had trouble with ranges of consecutive integers. Murmur Hash 3 is used
  by fastutil.
- Don't check keys for equality when re-inserting due to growing the
  table; the keys will already be unique.
- Remember the grow threshold instead of recomputing it on each insert

Also added unit tests for size estimation for specialized hash sets and maps.