This overhauls the very meager docs that currently exist to clarify
various understandable confusions that I've noticed, e.g. people look in
`std::raw` for the "real" types of slices like `&[T]`, or think that
`Slice<T>` refers to `[T]` (fixes#22214).
This patch takes the liberty of offering some "style" guidance around
`raw::Slice`, since there's more restricted ways to duplicate all
functionality connected to it: `std::slice::from_raw_parts{,_mut}` for
construction and `.as_{,mut_}ptr` & `.len` for deconstruction.
It also deprecates the `std::raw::Closure` type which is now useless for
non-type-erased closures, and replaced by `TraitObject` for `&Fn`, `&mut
FnMut` etc, so I guess it should be called a:
[breaking-change]
5e3ae102db