- 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.

This also removes the overlong link that failed tidy xD.

They closely mirror the docs for `copy_nonoverlapping`

The enumerated list of conditions is replaced by an explanation that
rust doesn't have a formal memory model. It does say that pointers
created directly from references are guaranteed to be valid, and links
to both the "Unsafe Code" section of the book and the "Undefined
Behavior" section of the reference.

Uses `x.offset(i)` must be valid for all `i` in `0..count`.

This splits "valid" into "valid for reads" and "valid for writes", and
also adds the concept of operation size to validity. Now functions which
operate on sequences state that e.g. pointer args must be "valid for reads of
size x".

Also rewrites the reads/writes section to be less reliant on `*const`,
`*mut`