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.

postgrest.cabal

@@ -352,14 +352,11 @@ test-suite observability
 test-suite doctests
   type:               exitcode-stdio-1.0
   default-language:   Haskell2010
-  default-extensions: OverloadedStrings
-                      NoImplicitPrelude
   hs-source-dirs:     test/doc
   main-is:            Main.hs
   build-depends:      base              >= 4.9 && < 4.22
-                    , doctest           >= 0.8
+                    , doctest-parallel  >= 0.4
                     , postgrest
                     , pretty-simple
-                    , protolude         >= 0.3.1 && < 0.4
   ghc-options:        -threaded -O0 -Werror -Wall -fwarn-identities
                       -fno-spec-constr -optP-Wno-nonportable-include-path