New lint: casting &mut T to &mut MaybeUninit<T> and exposing it as safe

[Kixunil]

As discussed at several different places, the pattern of casting &mut T to &mut MaybeUninit<T> or &mut [T] to &mut [MaybeUninit<T>] and exposing it as safe is reoccurring (see bellow), so making Correctness lint would help.

IDK if there's any policy about clippy suggesting particular crates, but if this is allowed, maybe suggest possibly_uninit.

• Doc issue at rust
• bytes is unsound
• uninit crate is unsound
• possibly_uninit was almost made unsound but I realized it and fixed it before it was too late. :)

[Manishearth]

Yeah, we should definitely suggest a crate for this.