Auto merge of #53508 - japaric:maybe-uninit, r=RalfJung

Auto merge of #53508 - japaric:maybe-uninit, r=RalfJung

Implement `MaybeUninit`

This PR:

- Adds `MaybeUninit` (see #53491) to `{core,std}::mem`.
- Makes `mem::{uninitialized,zeroed}` panic when they are used to instantiate an uninhabited type.
- Does *not* deprecate `mem::{uninitialized,zeroed}` just yet. As per https://github.com/rust-lang/rust/issues/53491#issuecomment-414147666, we should not deprecate them until `MaybeUninit` is stabilized.
- It replaces uses of `mem::{uninitialized,zeroed}` in core and alloc with `MaybeUninit`.

There are still several instances of `mem::{uninitialized,zeroed}` in `std` that *this* PR doesn't address.

r? @RalfJung
cc @eddyb you may want to look at the new panicking logic