don't show leading foralls in type signatures

[rwbarton@…]

When using ScopedTypeVariables it's necessary to write leading foralls in type signatures in order to scope free variables over the body of the function definition. Haddock reproduces these explicit foralls in its output. As I see it, whether a function definition is written with explicit foralls is an implementation detail--it doesn't affect users of the function, and in most cases the function could have been written without ScopedTypeVariables. So I think Haddock's output ideally shouldn't include these leading foralls; they're distracting, especially when some but not all of the functions in a module have them.