Add bit interleaving optimizations

These optimizations speed up encoding from 7 million
to 1.7 billion encodes per second. (Decodes go from 10 million
to the same 1.7 billion decodes per second.)

Cleaned up to fit the new code organization from
https://github.com/yinqiwen/geohash-int/commit/8840c4c8308f30e4a1ee8930ef35c298c60fd2f0