Back-porting pattern synonym type signature syntax for GHC 7.8.4 [Re: ANNOUNCE: GHC 7.8.4 Release Candidate 1]

[Dr. ERDI Gergo]

On Wed, 26 Nov 2014, Simon Peyton Jones wrote:

> My instinct is that (a)-(c) overwhelm (d); i.e. let sleeping dogs lie. 
> But what do others think?

Just to give an idea of how limited the scope of this change would be, 
I've went and implemented it, on the 'wip/pattern-synonym-sig-backport'
branch (of both GHC and Haddock).