Prepare modules for parser split.

We have to generalise the Doc (now DocH) slightly to remove the
dependency on GHC-supplied type.
Move parser + parser tests out to own package.

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.