Fixes and improvements to IntObjectHashMap

- Rewrite with linear probing, no state array, compaction at cleanup
- Optimize keys() and values() to not use reflection
- Optimize hashCode() and equals() for efficient iteration
- Fixed equals() to not return true for equals(null)
- Optimize iterator to not allocate new Entry at each next()
- Added toString()
- Added some new unit tests
Aww, I was running only the netty-comon tests because the full test suite failed for me since first checkout. The bug here is more subtle, I'm forgetting to handle the null vs. NULL_VALUE stuff. Fixing now..