Substructural typing for Phi stuff

[andrew-johnson-4]

type phi nodrop PointerLifetime = Live | Dead;

Here, for this Phi type, we specify with "nodrop" that the Phi type should not be removed by simply weakening a conjugate type during unification. This mostly affects library development, where accidentally dropping this type would cause a soundness hole.

[andrew-johnson-4]

type phi nodrop SafePointer = Live | Dead;

let unsafe-weaken(x: t[]+Phi<SafePointer::Weaken>): t[];

These objects are still "just pointers", but they now have ownership tracking thanks to the attached Phi type.