new compilerplugin returnbyref

Find places where we are returning a pointer to something, where we can
be returning a reference.
e.g.
class A {
struct X x;
public X* getX() { return &x; }
}
which can be:
public X& getX() { return x; }

Change-Id: I796fd23fd36a18aedf6e36bc28f8fab4f518c6c7