Stabilize mutable slice API

This commit is another in the series of vector slice API
stabilization. The focus here is the *mutable* slice API.

Largely, this API inherits the stability attributes [previously
assigned](rust-lang#16332) to the analogous
methods on immutable slides. It also adds comments to a few `unstable`
attributes that were previously missing them.

In addition, the commit adds several `_mut` variants of APIs that were
missing:

- `init_mut`
- `head_mut`
- `tail_mut`
- `splitn_mut`
- `rsplitn_mut`

Some of the unsafe APIs -- `unsafe_set`, `init_elem`, and `copy_memory`
-- were deprecated in favor of working through `as_mut_ptr`, to simplify
the API surface.

Due to deprecations, this is a:

[breaking-change]