Validating with Haddock

[Mateusz Kowalczyk]

Hi all,

I have now merged in the new parser and new features onto a single
branch. I'm having some issues validating with HEAD at the moment
(#8661, unrelated problem) but while I get that sorted out, someone
might want to try validating with Haddock changes on their own platform.

The full branch is at [1]. I have squashed the changes to what I feel is
the minimum number of commits until they completely stop making sense.
It should apply cleanly on top of current Haddock master branch. The
documentation is updated so you can read about what changed. Feel free
to ask any questions.

I will post again once I can confirm that the branch validates for me
without any new test failures.

Thanks for your patience.

[1]: https://github.com/Fuuzetsu/haddock/tree/new-features

-- 
Mateusz K.