Add back IntObjectMap.values(Class<V>)

Motivation:

Although the new IntObjectMap.values() that returns Collection is
useful, the removed values(Class<V>) that returns an array is also
useful. It's also good for backward compatibility.

Modifications:

- Add IntObjectMap.values(Class<V>) back
- Miscellaneous improvements
  - Cache the collection returned by IntObjectHashMap.values()
  - Inspector warnings
- Update the IntObjectHashMapTest to test both values()

Result:

- Backward compatibility
- Potential performance improvement of values()