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.

.github/workflows/test.yaml

@@ -63,7 +63,7 @@ jobs:
 
       - name: Run doctests
         if: always()
-        run: postgrest-test-doctests
+        run: nix-shell --run postgrest-test-doctests
 
       - name: Check the spec tests for idempotence
         if: always()