Fix #429: move from `cabal-doctest` to `cabal repl -w doctest`

[andreasabel]

Fix #429: move from cabal-doctest to cabal repl -w doctest.

• Remove doctests suite and thus the cabal-doctest dependency and the custom setup.
• Doctests can be run via make doctest, which will install doctest per GHC version and invoke cabal repl -w doctest.
• The extra -Wno-type-defaults ghc option is needed for doctest to not stumble over OverloadedStrings. See:
  • https://github.com/sol/doctest/issues/390
• The Haskell CI needs to be patched to add the doctests step, and certain RTS settings have to be removed for cabal repl -w doctest to work in this environment.