Shortened method syntax can now be used in constructors

This used to raise an error "cannot return expression from constructor", but it's now supported:

---
struct Number
{
    int x;

    void vf(int);
    this(int x) => vf(x);
    this(float x) => this(cast(int) x);
}
---

The expression body must be a `this`/`super` call or have type `void`.

Postblits and destructors already supported shortened method syntax because they return `void`.