core: Apply stability attributes to ptr mod

* null and mut_null are unstable. Their names may change if the unsafe
  pointer types change.
* copy_memory and copy_overlapping_memory are unstable. We think they
  aren't going to change.
* set_memory and zero_memory are experimental. Both the names and
  the semantics are under question.
* swap and replace are unstable and probably won't change.
* read is unstable, probably won't change
* read_and_zero is experimental. It's necessity is in doubt.
* mem::overwrite is now called ptr::write to match read and is
  unstable. mem::overwrite is now deprecated
* array_each, array_each_with_len, buf_len, and position are
  all deprecated because they use old style iteration and their
  utility is generally under question.
auto merge of #14529 : brson/rust/ptr, r=brson

This time we're not promoting anything directly to 'stable', but instead promoting functions we're happy with to 'unstable'. They'll become stable in another pass later.

* null and mut_null are unstable. Their names may change if the unsafe
  pointer types change.
* copy_memory and copy_overlapping_memory are unstable. We think they
  aren't going to change.
* set_memory and zero_memory are experimental. Both the names and
  the semantics are under question.
* swap and replace are unstable and probably won't change.
* read is unstable, probably won't change
* read_and_zero is experimental. It's necessity is in doubt.
* mem::overwrite is now called ptr::write to match read and is
  unstable. mem::overwrite is now deprecated
* array_each, array_each_with_len, buf_len, and position are
  all deprecated because they use old style iteration and their
  utility is generally under question.

Note that `mem::overwrite`, which was just declared stable last week, is deprecated now in favor of `ptr::write`. Woo!