HsWrappers are horribly inconsistent at the moment. I intended that
  WpLam, WpApp     are for evidence abstraction/application
  WpTyLam, WpTyApp are for type abstraction/application

But when we zonk (WpApp co), where co is a coercion variable, we 
get a *coercion* not a coercion *variable*.   So for now I'm making
it into a WpTyApp, which the desugarer handles perfectly well.

(I'd forgotten to zonk it properly at all; that is the bug that 
this patch fixes.)