Deprecate -Zmiri-allow-ptr-int-transmute?

[RalfJung]

I would love to deprecate and remove -Zmiri-allow-ptr-int-transmute sooner rather than later. That would let us do some nice cleanup in the interpreter: we could remove the ptr_from_addr_transmute hook, which means scalar_to_ptr does not need access to the InterpCx any more, so we can just make it a method on Scalar.