staging: Add Snappy compression library (v3)

Google's Snappy data compression library is a faster alternative to LZO,
optimized for x86-64. On compressible input it compresses ~2.5x faster than LZO
and decompresses ~1.5-2x faster than LZO. On incompressible input, it skips the
input at 100x faster than LZO and decompresses ~4x faster than LZO.
It is released under BSD license.
This is a kernel port from user space C++ code.
The current intended use is with zram (see next patch in series).

Signed-off-by: Zeev Tarantov <zeev.tarantov@gmail.com>