Tracking issue for `#![feature(maybe_uninit_extra,const_maybe_uninit

Tracking issue for `#![feature(maybe_uninit_extra,const_maybe_uninit_write)]`

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

This issue specifically tracks the following unstable methods:

impl<T> MaybeUninit<T> {
    pub unsafe fn assume_init_read(&self) -> T { ... }
    pub unsafe fn assume_init_drop(&mut self) { ... }
}

and it tracks const-ness of

impl<T> MaybeUninit<T> {
    pub const fn write(&mut self, val: T) -> &mut T { ... }
}