We only have one version of tahoe-lafs here right now anyway, so the
identifier isn't ambiguous without a version number, and there are probably
more changes required to really support more than one version at a time.

as Tahoe now wants, so it can respect available space when allocating more for
a lease

Also limit build concurrency to 8.  A lot of the tests are I/O bound so they
are bottlenecked and extra cores don't help.

It was consistent so it worked fine but it was misleading since it points at
21.05, not the non-existent 20.15.

Defend against the length-contraction slot_testv_and_readv_and_writev DoS

Implement a minimal interface for reporting spent ZKAPs to a spending service.

Fix certificate validation errors during environment setup on CI

maybe this nix-env invocation will work

hopefully nix-shell behaves a little more predictably than nix-env wrt NIX_PATH

maybe 2.3.16, a very recent 2.3, will do better.

that version we were on was pretty darn old, for sure

Handle passes using the `Pass` model object in more places

Make generating cross-plaform pins easier.

Convert `Pass`, `UnblindedToken`, `RandomToken` and `Voucher` to use bytes internally.

The later function takes a count of passes to generated, and passes
them to `make_passes`. This also changes `make_passes` to return `Pass`es,
which is what `get_passes` wants to receive.

Record server-side Prometheus metric for spent ZKAPs