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.

Fix metric names

Update to a newer Haskell.nix and materialize the Nix expressions for PaymentServer

This reduces the amount of memory required to install PaymentServer by ~500MB.

It's called "side effects".
Look it up.

Remove the Stack-based steps from CI, leaving only the Nix-based steps

Only use Nix.  I can figure out how to cache the Nix stuff on CircleCI.  The
stack stuff is hard and not obviously worth the effort.