Support arbitrary self types

[tmandry]

Support self types like &Box<Self> and Arc<Self>, and Pin<Box<Self>>.

These shouldn't actually be "arbitrary" because we need to know that we can coerce their inner types to dyn.. the same requirements that would need to be verified for rust-lang/rfcs#3621 (#[derive(CoercePointee)] as it's now being called). For self types involving Pin, we would also need to know the PinCoerceUnsized trait detailed in that RFC is implemented. All of this is unstable for now, so we can support it in a later PR.

For now, we can just allowlist specific pointer types from the standard library.

[spastorino]

There was an attempt that didn't work here #15