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.

cabal.project

@@ -2,3 +2,6 @@ packages: postgrest.cabal
 tests: true
 allow-newer:
   hasql:postgresql-libpq
+
+-- https://github.com/martijnbastiaan/doctest-parallel/blob/main/example/README.md#cabalproject
+write-ghc-environment-files: always