Some cleanups to rapidhash.

 - Remove all #ifdefs that we don't need.
 - Add some comments.
 - Add #include guards.
 - Replace in/out pointer manipulation in rapid_mum with multi-argument
   return.
 - Add a fuzzer.

Change-Id: I21762041b8db703be5420158ff8164dbba881cd6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5698845
Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Steinar H Gunderson <sesse@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326903}