Values support in call, callwith

[Technologicat]

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.