Add helper functions for Array<MaybeUninit<T>, U>

[tarcieri]

Adds the following functions:

• uninit: construct an uninitialized Array<MaybeUninit<T>, U>
• assume_init: converts to Array<T, U> assuming all elements are initialized

[newpavlov]

To be safe I think we should ask on IRLO about the uninit method, but AFAIK the code should be fine. It's worth to expand the safety comment and explicitly note that we "assume init" an array of MaybeUninits.

[tarcieri]

Expanding the safety comment sounds good. I can also add a link to the similarly-shaped uninit_array method.

[tarcieri]

@newpavlov d7f32a4 should make it much more explicit what's happening (at the cost of slightly more complicated bounds), and also clears the clippy warning

[tarcieri]

Seems at little closer to pub const fn assume_init too:

error[E0658]: cannot borrow here, since the borrowed element may contain interior mutability
   --> src/lib.rs:245:25
    |
245 |         Array(ptr::read(self.0.as_ptr().cast()))
    |                         ^^^^^^
    |
    = note: see issue #80384 <https://github.com/rust-lang/rust/issues/80384> for more information