Generalize Ptr invariant relaxation

[jswrenn]

This builds on machinery introduced in #1166.

It would be nice to introduce a general invariant relaxation method in the form:

fn relax_invariants<J>(self) -> Ptr<'a, T, J>
where
    I: invariants::AtLeast<J>,
{
    // SAFETY: `I` is at least as restrictive as `J`.
    unsafe { Ptr::from_ptr(self) }
}

...and call it in our specific .forget_* methods.

We think this might be blocked on stabilizing feature(marker_trait_attr) (rust-lang/rust#29864).