Allow default constructors for structs

[Richard (Rikki) Andrew Cattermole]

There is a subset of constructors we certainly can do without analyzing 
type states and the implications thereof.

Specifically, if you have a constructors with only default values, 
require that it must have at least one named argument into it.

Anything that messes with .init, is going to need a lot more work than 
presented here.