Values support in call, callwith

For completeness, the `call` and `callwith` functions should support the `Values` abstraction, for unpacking multiple and/or named return values into function arguments. The function composition utilities already support this.

Instead of `*a` and `**kw`, if given exactly one positional argument `x`, and `type(x) is Values`, then unpack it, thus populating `*a` and `**kw`.
