Redundant quantification when rendering GADT constructors from external packages

[RyanGlScott]

(Yet another issue noticed in well-typed/generics-sop#92.)

Run Haddock on this code:

module Bug ((:~:)(..)) where

import Data.Type.Equality

And you'll get this:

Notice that the type signature for Refl quantifies b, which is completely redundant! The culprit is these lines of code:

haddock/haddock-api/src/Haddock/Convert.hs

Lines 337 to 342 in fd56ac4

	if use_gadt_syntax
	then return $ noLoc $
	ConDeclGADT { con_g_ext = noExt
	, con_names = [name]
	, con_forall = noLoc True
	, con_qvars = synifyTyVars (univ_tvs ++ ex_tvs)

This is constructing a type signature for a GADT constructor by displaying all of the universally quantified type variables following by the existentially quantified type variables. But this is not always what the user has written, as shown in the example above. We really ought to be using dataConUserTyVars instead, which takes this information into account.

Patch incoming.

[RyanGlScott]

Fixed in #1022.