core: Stabilize the mem module

Excluding the functions inherited from the cast module last week (with marked
stability levels), these functions received the following treatment.

* size_of/size_of_val - these methods have become #[stable]
* nonzero_size_of/nonzero_size_of_val - these methods have been removed
* min_align_of/min_align_of_val - these methods are now #[stable]
* pref_align_of/pref_align_of_val - these methods have been renamed without the
  `pref_` prefix, they are the "default alignment" now. This decision is in line
  with what clang does (see url linked in comment on function). Both functions
  are now #[stable]
* init - renamed to zeroed and marked #[stable]
* uninit - marked #[stable]
* move_val_init - renamed to overwrite and marked #[stable]
* {from,to}_{be,le}{16,32,64} - all functions marked #[stable]
* swap/replace/drop - marked #[stable]

[breaking-change]