Hashmap revision

These patches rewrite portions of Crafty's Hashmap.  Changes:

* Uses the bit pattern `(i << 16)^j`as a coordinate key instead of the string `i + SPACE + j`.  This is faster and creates less garbage, but would break a scene that involved more than 65k cells.  (With the default settings, that would be 2 million pixels.)  I don't this is a real concern, but enlighten me if I'm wrong!  :)

* Add a function to refresh an objects position in the hashmap, rather than removing it and then adding it back.  The old way created a lot of garbage.  (I imagine the refresh function could be further optimized.)

* Where possible, avoid creating objects by reusing existing ones.  Every use of mbr() makes Firefox's GC cry, and we don't want that.

I've used these changes for a long time in a game I wrote, so they seem fairly bug free.  My own benchmarking showed a general performance improvement.   I ran some jsperf tests on [encoding](http://jsperf.com/string-vs-bit-hash/2) and [decoding](http://jsperf.com/hash-reversal-for-bits-and-strings) the hash, and the bitwise version won out with both, but the most noticeable gain was the reduction in GC pauses in Firefox.