Ubuntu 20.04 is now the oldest version of Ubuntu still within support.

This makes it slightly clearer what the purpose of this binary is, and also will
be more consistent with some upcoming planned tools.

Github: related to #167 "switch all Python dependencies to Python 3 in
  packaging/debian"

Github: related to ##146 "fails to build on Fedora 30"

The Unlicense comes with the added benefit of a disclaimer of liability and
warranty.

Rumur is reasonably usable now.

This lets us revert to a completely public domain licence which is much nicer.

Related to Github #61 "use infinite precision numbers in the generator"

These allow for nicer presentation for mathematicians.

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.

More to come...