Remove old syntax for struct constructors

[erickt]

The old syntax for struct constructors is scheduled to be removed:

struct S {
    x: int,
    new() { self.x = 5 }
}

It should be replaced with:

struct S {
    x: int
}

fn S() {
    S { x: 5 }
}

[catamorphism]

Fixed in b4e547d