lib: add cityhash function

CityHash64 appears to be the fastest available block hashing algorithm
that is good enough for dedupe.  It takes much less CPU than the CRC64
function, and avoids hash-collision problems with file formats that use
CRC64 as an integrity check on 4K block boundaries.

Extracted from git://github.com/google/cityhash with the "CRC" hash
functions (which require Intel/AMD CPU support) removed.  We don't
need those, and they introduce a new (if only theoretical) build-time
dependency.

Signed-off-by: Zygo Blaxell <bees@furryterror.org>

lib/Makefile

@@ -5,6 +5,7 @@ default: libcrucible.a
 
 CRUCIBLE_OBJS = \
 	chatter.o \
+	city.o \
 	cleanup.o \
 	crc64.o \
 	error.o \