split off RQ encoding steps to another file

Summary: After Alexandr's optimizations the ResidualQuantizer code has become harder to read. Split off the quantization code to a separate .h / .cpp to make it clearer.

Differential Revision: D48448614

fbshipit-source-id: 1c04970414ebe8d0e65be3d4117e39d809e54e10

Author: mdouze