Add inherent versions of MaybeUninit methods for slices

[clarfonthey]

This is my attempt to un-stall #63569 and #79995, by creating methods that mirror the existing MaybeUninit API:

impl<T> MaybeUninit<T> {
    pub fn write(&mut self, value: T) -> &mut T;
    pub unsafe fn assume_init_drop(&mut self);
    pub unsafe fn assume_init_ref(&self) -> &T;
    pub unsafe fn assume_init_mut(&mut self) -> &mut T;
}

Offering these new methods:

impl<T> [MaybeUninit<T>] {
    pub fn write_copy_of_slice(&mut self, value: &[T]) -> &mut [T] where T: Copy;
    pub fn write_clone_of_slice(&mut self, value: &[T]) -> &mut [T] where T: Clone;
    pub unsafe fn assume_init_drop(&mut self);
    pub unsafe fn assume_init_ref(&self) -> &[T];
    pub unsafe fn assume_init_mut(&mut self) -> &mut [T];
}

Since the assume_init methods are identical to those on non-slices, they feel pretty natural. The main issue with the write methods is naming, as discussed in #79995 among other places. My rationale:

• The term "write" should be in them somewhere, to mirror the other API, and this pretty much automatically makes them not collide with any other inherent slice methods.
• I chose write_clone_of_slice and write_copy_of_slice since clone and copy are being used as nouns here, whereas they're being used as verbs in clone_from_slice and copy_from_slice.

I'm hoping these don't require an ACP since they're just refining upon existing APIs, but I can write one up if you prefer.

[rustbot]

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[tgross35]

A test for panic during clone would probably be good to verify expected behavior

[tgross35]

The assume_init_* methods could probably use examples.

[tgross35]

The implementation looks pretty reasonable to me, but as this is an API decision:

r? libs-api

Is the intent here to replace the existing copy_from_slice and clone_from_slice? If so, those should be removed.

[clarfonthey]

I decided to not delete them since so many people are using these methods, but I can deprecate them depending on what people think is best.

[rust-log-analyzer]

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[Amanieu]

We discussed this in the libs-api meeting. We're happy with these changes, but we would also like to see similar changes for the fill (#117428) and as_bytes (#93092) methods so that all slice-related methods are turned into inherent methods. Note that fill will need to be renamed since it conflicts with the generic fill method on slices (perhaps fill_init?).

Also, the old methods on MaybeUninit should be removed (in a separate PR) so that we don't have duplicate methods that do the same thing.