LZ4 partial library source

The following files were pulled from: 
    https://github.com/lz4/lz4
at commit d443718 (tag v1.9.3)

lib/lz4.c
lib/lz4.h
lib/lz4hc.c
lib/lz4hc.h

The remaining files are unnecessary since we don't use the lz4 framing format.

