Discussing the validity invariant of raw pointers.

For pointers to sized types, this should probably be the same as the invariant for usize -- see the integer topic for discussing whether uninitialized bits are allowed or not.

For pointers to unsized types, there is an additional question: to what extent does the metadata have to be initialized/valid? Do we require it to be "valid" enough to determine size and alignment, e.g. do we require that the vtable pointer actually point to allocated memory?