Fix linting of unused type variables - #3464

Fixes #3380

The original algorithm didn't appear to be caring about the scope of type variables introduced with forall - it just asked for "all the type variables" that appear in some type.

This PR changes this so it walks the type and respects variable scope.

With the following code:

type X a =
  { a :: forall f. f a
  , b :: forall f. a
  }

type Y a =
  { a :: forall g. g a
  , b :: forall h. a
  }

Output diff (trimmed):

+   Type variable f is ambiguous...
    Type variable h is ambiguous...