Rollup merge of #75392 - TimDiekmann:non-null-uninit-slice, r=RalfJung

Rollup merge of #75392 - TimDiekmann:non-null-uninit-slice, r=RalfJung

Add `as_uninit`-like methods to pointer types and unify documentation of `as_ref` methods

This adds a convenient method to retrieve a `&(mut) [MaybeUninit<T>]` from slice pointers (`*const [T]`, `*mut [T]`, `NonNull<[T]>`). See also https://github.com/rust-lang/wg-allocators/issues/66#issuecomment-671789105.

~I'll add a tracking issue as soon as it's reviewed and CI passed.~
Tracking Issue: #75402

r? @RalfJung