Support calling pointer-receiver methods on struct fields

[tchajed]

Right now we write code like

type foo struct {
    bar *Bar
}

func (f *foo) UseBar() {
    f.bar.DoSomething()
}

This results in double dereferencing - once to get the pointer bar and another within DoSomething() to access the fields of Bar.

Idiomatic Go would use bar Bar and then call pointer methods through the foo pointer. In GooseLang we already support constructing the pointer using a location offset, but goose doesn't emit code that takes advantage of this. See failing_testFooBarMutation for an example of what we'd like to support.

Implementing this probably requires some refactoring of the address-of code, to factor out all the code that computes pointers.

[RalfJung]

From the title this sounds like a duplicate of #1?