Allocate keys into off-heap memory

[dgrnbrg]

Keys are never GCed (although they can be in certain occaisions).

It'd be better to store just hashes in the chm, and do the collision detection/chaining off-heap.

[dgrnbrg]

This will be easy to implement once #15 is done.

[dgrnbrg]

The key to this implementation is that BinaryKey must have 2 constructors: a direct and a nondirect one. The direct one should be used on inserts into the hashtable, and the nondirect one for comparisons. In order to facilitate reuse of direct buffers, every put must first try to reuse the existing equivalent direct buffer already in the chm.