Factor out the productions for functor parameters.

Functor parameters can occur in three contexts:

  (1) in functor bindings:

        module M (X : S) (Y : T) = ...

  (2) in anonymous functor expressionms:

        functor (X : S) (Y : T) -> ...

  (3) in signature expressions

        module type T = functor (X : S) (Y : T) -> ...

This patch simplifies the grammar by using common productions
(functor_arg, functor_args) to handle all three contexts, and by
combining the productions for named and unnamed parameters.

As a side effect, unnamed functor arguments are now available in
functor bindings:

     module M (_ : S) (_ : T) = ...