Support type inference for named constructors

[jvasileff]

For example:

shared class Box<Element> {
    shared new Of(Element e) {}
}

Box<String> bs = Box.Of("");
// Missing type arguments to generic declaration:
//  Box declares type parameters Element

[gavinking]

I have implemented this on a branch. It was most natural to also support inference for qualifying types in static method references, for example, List.get([1,2,3])(0).

But that means I still have to really think this through and see if I can poke holes in it.

[gavinking]

OK, so this is now fully implemented, and the branch merged. I'm leaving the issue open for now, because this needs to be documented in the spec.

[gavinking]

Spec done.