saxtons probably has everything we need so nothing seems to get satisfied from
iohk, let's make sure we can actually satisfy anything at all from iohk

https://nixos.org/manual/nix/stable/command-ref/conf-file.html says there's a
--extra-substituters but nix-build seems to disagree.  maybe a nix/nix-build thing.

$NIX_CONFIG is a nix >=2.4 feature so it doesn't help w/ 2.3

The Nix images have /bin/sh instead so it probably doesn't work

Update to a libchallenge_bypass_ristretto_ffi with correct pkg-config

it depended on now-deleted nix source file
make it use niv sources instead
also drop the NIXPKGS_REV which has been unused for a while

Add a tool for extracting the public key from a signing key

Update haskell.nix version used in CI and deployment.

Prefix all the metric names with "payment_"

Enforce a certain number of tokens on redemption

Also reduce duplication in the test implementation a bit
And add some more error case tests
And remove the old, no-longer used redemption parameter globals
Some Integers becomes Ints for intra-module consistency

With the other changes in this PR, a fresh build of all dependencies is <15m,
so we don't need a large instance. However, the instances *do* have multiple
cores, so take advantage of that.

Update the README to reflect PaymentServer's new API.