Fix scoped type variables for expression type signatures

Fix scoped type variables for expression type signatures

I had forgotten to bring scoped type variables into scope at an expression
type signature, such as
	e :: forall s. <type>
where 's' should scope over the expression e.

Like everything to do with scoped type variables, fixing this took an
unreasonable amount of work.  I'm sure there must be a better way to
achitect this!

I updated the user manual too.

A test is tc213.

It would be good to push this into 6.6.1