I get rid of the Monoid instance because we weren't satisfying the laws.
Convenience of having <> didn't outweigh the shock-factor of having it
behave badly.

The benefit of this is that the ‘top-level’ element of such lists is
properly wrapped in <p> tags so any CSS working with these will be
applied properly. It also just makes more sense.

Pointed out at jgm/pandoc#1346.

I can not remember why they were wrapped in paragraphs to begin with and
it seems unnecessary now that I test it. Closes #307.

They are much more useful to the users here.

We move some types out that are necessary as well and then
re-export and specialise them in the core Haddock.

Reason for moving out spec tests is that if we're working on the parser,
we can simply work on that and we can ignore the rest of Haddock. The
downside is that it's a little inconvenient if at the end of the day we
want to see that everything passes.