- Add links to the GNU libc docs for `memmove`, `memcpy`, and
`memset`, as well as internally linking to other functions in `std::ptr`
- List invariants which, when violated, cause UB for all functions
- Add example to `ptr::drop_in_place` and compares it to `ptr::read`.
- Add examples which more closely mirror real world uses for the
functions in `std::ptr`. Also, move the reimplementation of `mem::swap`
to the examples of `ptr::read` and use a more interesting example for
`copy_nonoverlapping`.
- Change module level description
- Define what constitutes a "valid" pointer.
- Centralize discussion of ownership of bitwise copies in `ptr::read` and
provide an example.