Rewrite docs for `std::ptr`

- 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.

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