doctest wip

[sordina]

Some examples do work:

> cabal exec -- doctest -isrc -XLambdaCase -XUndecidableInstances -XFunctionalDependencies src/Control/Category/Cartesian.hs
Warning: ignoring unrecognised input `.'
Examples: 25  Tried: 25  Errors: 0  Failures: 0

However, currently running into this bug:

> cabal exec -- cabal test
Build profile: -w ghc-9.2.5 -O1
In order, the following will be built (use -v for more details):
 - monoidal-functors-0.2.1.0 (test:monoidal-functors-test) (first run)
Preprocessing test suite 'monoidal-functors-test' for monoidal-functors-0.2.1.0..
Building test suite 'monoidal-functors-test' for monoidal-functors-0.2.1.0..
Loaded package environment from /Users/lyndon/code/monoidal-functors/dist-newstyle/tmp/environment.-18821/.ghc.environment.x86_64-darwin-9.2.5
Loaded package environment from /Users/lyndon/code/monoidal-functors/dist-newstyle/tmp/environment.-18821/.ghc.environment.x86_64-darwin-9.2.5
Running 1 test suites...
Test suite monoidal-functors-test: RUNNING...
Warning: ignoring unrecognised input `.'

src/Data/Bifunctor/BiInvariant.hs:110:65: error:ghc-9.2.5: panic! (the 'impossible' happened)
  (GHC version 9.2.5:
	No skolem info:
  [k1_a9lA]
  Call stack:
      CallStack (from HasCallStack):
        callStackDoc, called at compiler/GHC/Utils/Panic.hs:181:37 in ghc:GHC.Utils.Panic
        pprPanic, called at compiler/GHC/Tc/Errors.hs:2912:17 in ghc:GHC.Tc.Errors

Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug