This will make it easier to add the new allowed-public-keys item which must be
present regardless of chosen redeemer.

Now PaymentController has a set of these as an attribute.  It could possibly
consider making some decisions based on this value now.

We're going to start looking at it.  It's not reasonable for its value to be None.

Disregard element order when checking invalid pass information

The order of the passes only comes from Python dictionary iteration order.  It is largely meaningless.

Add a `__str__` to MorePassesRequired

Set up Cachix cache to speed up builds (beyond even the point where CircleCI stops timing them out)

Co-authored-by: Tom Prince <tom.prince@hocat.ca>

We collect coverage info so we can't very easily run on multiple cores.  So we
mostly can't take advantage of the bigger machine.

With Cachix we can now cache even partial build successes so even if we lose
the whole cache and have to start from scratch, the build should now converge
on success even if it takes a few tries.

hope it does not break the build cachix setup step

Attempt to set up a scheduled NixOS job workflow to refresh the CircleCI cache

Also move this time to a point shortly in my future so it will run and I can
see what happens.

Account for leading and trailing whitespace around the Ristretto key when reading it from a file

This exercises whitespace and no-whitespace cases.

Remove the bundled Tahoe-LAFS and just require it as a param or nixpkgs attr

Also, always store the cache because it should at least save us some effort
next time.