Fix syntax of (using ..., Type)

[FlandiaYingman]

Fix #274.

This PR fixes

module M with
  fun f(using Int, Int) = 42

which used to parse the using Int part only but failed to parse the (second) Int part.

[LPTK]

Does the following syntaxes work? Do we have tests for them?

fun foo(using Int, arg: Str)

fun foo(using arg: Str, Int)

[FlandiaYingman]

Does the following syntaxes work? Do we have tests for them?

fun foo(using Int, arg: Str)

fun foo(using arg: Str, Int)

They work fine and I just added them to the tests.

[LPTK]

LGTM!