switch to using xxHash for hashing states

By using a proper hashing function we get a fairly dramatic speed up. This now
makes checking long-running2 practical. We get 453 seconds on our current
representative platform. Note, this is still single-threaded.

This commit contains sources from xxHash v0.6.5. xxhash.c from that release has
been pasted into xxhash.h so that we can emit the checker as a single source
file.