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.

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

... on purpose so devs know what to expect from users like me.

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

It is easy to erroneously add extra white space to a text file. For example when writing with an editor like `nano` or when using `echo` without the `-n` argument.

Handing that white space over to python-challenge-bypass-ristretto's `decode_base64()` method will make it fail in a rather opaque way, see https://github.com/LeastAuthority/python-challenge-bypass-ristretto/issues/37.

This eliminates white space including newlines around the key expected to be in that file.

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.