Uniquable RdrName instance

[Jan Stolarek]

> Why do you want this? The danger I see lurking is that you might find two
> RdrNames that give the same answer to getUnique but represent different
Yes, that's exactly my concern.

> Why do you want this?
I'm working on #7282 - RebindableSyntax and Arrows. Here's a snippet from desugaring of arrow 
notation:

leaves = concatMap leavesMatch matches
 where
   leavesMatch :: LMatch Id (Located (body Id)) -> [(Located (body Id), IdSet)]
   leavesMatch (L _ (Match pats _ (GRHSs grhss binds)))
         = let defined_vars = mkVarSet (collectPatsBinders pats) `unionVarSet`
                                          mkVarSet (collectLocalBinders binds)
             in  [(body, mkVarSet (collectLStmtsBinders stmts) `unionVarSet` defined_vars) | L _ 
(GRHS stmts body) <- grhss]

This is in the desugarer. But I need to also know `length leaves` at the renaming stage, which 
means I need to call `concatMap leavesMatch matches`. The problem is that in the renamer my 
datatypes are not parametrised by `Id`. I turned `Id` into a type parameter `id`, but then I 
can't use VarSets. I could use more general UniqueSets but only if `id` type parameter is an 
instance of Uniquable. And since in the renamer the datatypes are parametrized with RdrName this 
leads me to wanting Uniqable RdrName instance.

Janek