chore: run doctests in parallel

Runs the doctests much faster, which is potentially useful in
combination with postgrest-watch for local development.

This implies that doctests run on compiled code, not in a GHCi session,
which has some implications:
- Only exported functions can be tested.
- Imports need to be made explicit in doctests themselves.

On the flipside, this would allow us to potentially include doctest
results in code coverage, I believe.

This change is a requirement to vendor hasql, which otherwise breaks the
existing doctests: hasql contains a .hsc file, which *needs* to be
compiled - not interpreted - to make the tests work.

nix/tools/tests.nix

@@ -55,8 +55,6 @@ let
         withEnv = postgrest.env;
       }
       ''
-        # This makes nix-env -iA tests.doctests.bin work.
-        export NIX_GHC=${postgrest.env.NIX_GHC}
         ${cabal-install}/bin/cabal v2-run ${devCabalOptions} test:doctests
       '';