split off RQ encoding steps to another file (facebookresearch#3011)

Summary:
Pull Request resolved: facebookresearch#3011

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: 9a067204915551d4aa99a4a77b4a976bab108349

faiss/CMakeLists.txt

@@ -72,6 +72,7 @@ set(FAISS_SRC
   impl/pq4_fast_scan.cpp
   impl/pq4_fast_scan_search_1.cpp
   impl/pq4_fast_scan_search_qbs.cpp
+  impl/residual_quantizer_encode_steps.cpp
   impl/io.cpp
   impl/lattice_Zn.cpp
   impl/NNDescent.cpp
@@ -171,6 +172,7 @@ set(FAISS_HEADERS
   impl/lattice_Zn.h
   impl/platform_macros.h
   impl/pq4_fast_scan.h
+  impl/residual_quantizer_encode_steps.h
   impl/simd_result_handlers.h
   impl/code_distance/code_distance.h
   impl/code_distance/code_distance-generic.h