mem::uninitialized -> MaybeUninit

[alecmocatta]

Now that MaybeUninit is stable and usage has been somewhat clarified, switch to it. The uninitialized byte buffer impl is copied from io::copy() and thus currently inherits:

// FIXME(#53491): This is calling `get_mut` and `get_ref` on an uninitialized
// `MaybeUninit`. Revisit this once we decided whether that is valid or not.
// This is still technically undefined behavior due to creating a reference
// to uninitialized data, but within libstd we can rely on more guarantees
// than if this code were in an external lib.