remove unsound MaybeUninitSlice::from_init_mut and useless `...::fr…

[WaffleLapkin]

…om_init`

Well, that's weird. I've had a safety comment:

// `MaybeUninit<T>` is guaranteed to have the same ABI as `T`, so
// it's safe to cast `&mut [T]` to `&mut [MaybeUninit<T>]`

But it's wrong. &mut T and T isn't the same thing. While it's true that T => MaybeUninit<T>
or the same for an owned container (array, box, etc) should be fine, for an unique borrowed
container (&mut _, &mut [_]) it is definitely not fine, because the original owned value
remains T. Example of such a UB in safe code:

let mut a = ["string"];
<_>::from_init_mut(&mut a[..])[0] = MaybeUninit::uninit();
println!("{}", a); // segfault

You can also think of MaybeUninit<T> as a supertype of T and
then note that &mut T is invariant over T: https://doc.rust-lang.org/nomicon/subtyping.html#variance

The weirdest part of all of this is that I haven't tested those functions.
There aren't any tests. There aren't any test for safe function with unsafe in it!

I am ashamed...

A related issue in rust-lang repo, about documenting unsoundness
of &mut T => &mut MaybeUninit<T>: rust-lang/rust#66699