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: 1ad471b163bbd19f63ef48cab8ba31ab8016b642

Author: mdouze