Add make doctest [ci skip]

This is poor man doctest.

Prerequisites:
- `doctest` executable is in `$PATH`, compiled with GHC used to
  new-build the Cabal
- `.ghc.environment` file is generated

As GHC picks up dependencies from `.ghc.environment` file, a simple
`doctest MODULES` works.

As you see, I had a bug in doctests. For now, I don't add this to CI,
as it's enough to run doctests occasionally (there are only 4 examples
atm).