hashmap: Use SSE to speed up hashmap

The original FNV hashing algorithm processes one byte at a time.  SSE
lets us process more.  This is a lackadaisically tuned implementation;
in my tests, it was the fastest by a hair.  It is about twice as fast
as the original FNV, for about a 5-10% speedup on git status (with
watchman).

Note that you are unlikely to see speed improvements in test-hashmap,
since it uses very short, very similar strings.

Signed-off-by: David Turner <dturner@twitter.com>