Support key prefix compression

[broneill]

The node encoding format already has placeholders for key compression, by managing prefixes up to 255 bytes in length. Finishing the implementation is a bit tricky when considering node splits and merges.

[broneill]

Implementation would make binary search code much more complicated. Considering that it's the primary bottleneck, slowing it down further is a bad idea. General page-level compression is simpler and yields better space reduction.

Also see issue #38.