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.

test/doc/Main.hs

@@ -1,27 +1,7 @@
-module Main (main) where
-
-import Test.DocTest (doctest)
-
-import Protolude
+module Main where
 
+import System.Environment (getArgs)
+import Test.DocTest       (mainFromCabal)
 
 main :: IO ()
-main =
-  doctest
-    [ "-XOverloadedStrings"
-    , "-XNoImplicitPrelude"
-    , "-XStandaloneDeriving"
-    , "-XDuplicateRecordFields"
-    , "-isrc"
-    , "src/PostgREST/ApiRequest/Preferences.hs"
-    , "src/PostgREST/ApiRequest/QueryParams.hs"
-    , "src/PostgREST/Config.hs"
-    , "src/PostgREST/Error.hs"
-    , "src/PostgREST/MediaType.hs"
-    , "src/PostgREST/Network.hs"
-    , "src/PostgREST/Plan.hs"
-    , "src/PostgREST/Query/SqlFragment.hs"
-    , "src/PostgREST/Response.hs"
-    , "src/PostgREST/Response/Performance.hs"
-    , "src/PostgREST/SchemaCache/Identifiers.hs"
-    ]
+main = mainFromCabal "postgrest" =<< getArgs