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.

The new binding completely obscures the old binding.  The `else` case was
recursive with no base.

This way we get the cache even if the test suite fails

Remove the vcpython27 installation step

Switch to the new get-pip.py URL

Add ``default-token-count`` configuration item

The redeemer will submit this many tokens with its redemption request now.  It
needs to agree with whatever the issuer wants to do, of course.

Introduce `direct_tahoe_configs` strategy

It builds _Configs instead of functions that return _Configs.

CircleCI-based macOS CI

Fixes: #99

Since I don't want to build another right now, drop it.

So optimistic...