staging: Add Snappy compression library
 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).