Model all memory operations in Ptr

[joshlf]

Our eventual goal for Ptr is that it encodes the complete taxonomy of (single-threaded) memory operations one might want to perform on values, references, containers, or other smart pointer types. When that is done, 100% of zerocopy should consist of uses of Ptr, mostly without the need for unsafe code.

This issue tracks various designs that will be required in order to reach that goal.