NEW TRACKING ISSUE = #63566

This is a tracking issue for the RFC "Deprecate uninitialized in favor of a new MaybeUninit type" (rust-lang/rfcs#1892).

Steps:

• Implement the RFC (cc @rust-lang/libs)
• Adjust documentation (in stabilize core parts of MaybeUninit #60445)
• Stabilization PR (in stabilize core parts of MaybeUninit #60445)

Unresolved questions:

• Should we have a safe setter that returns an &mut T?
• Should we rename MaybeUninit? (Rename MaybeUninit to MaybeUninitialized #56138)
• Should we rename into_inner? Should it be more like take instead and take &mut self?
• Should MaybeUninit<T> be Copy for T: Copy?
• Should we allow calling get_ref and get_mut (but not reading from the returned references) before data got initialized? (AKA: "Are references to uninitialized data insta-UB, or only UB when being read from?") Should we rename it similar to into_inner?
• Can we make into_inner (or whatever it ends up being called) panic when T is uninhabited, like mem::uninitialized does currently? (done)
• Seems like we want to not deprecate mem::zeroed. We should however remember to also update its documentation together with MaybeUninit, make sure people are aware that this is insta-UB if all-0-bits does not satisfy the type's validity invariant.