[Haddock] vim help text export

[Simon Hengel]

> It will reduce maintenance efforts because your backend does not have
> to be maintained as part of Haddock. So it will be you who maintains
> the backend instead of us :-) But if it's not convenient to write your
> backend as a separate program (because of the immaturity of the
> Haddock API for example), I will accept patches for Haddock.

Wouldn't it be neat to write one "backend" that produces an AST, lets
call it HaddockCore for now.  Than we could provide a catamorphism for
that, and writing a backend would just be folding over that AST.  I
think that would not only reduce maintenance costs and duplicate backend
code, but also help in writing correct code.  Currently it is relatively
easy to miss parts of the tree.

Finally we call that AST producing "backend" thing "mature Haddock API".**

Am I oversimplifying things?

Cheers,
Simon

** Or maybe better "mature but evolving" :D