Mem::uninitialized, `!` and trap representations

language design
18

I think we can solve the SmallVec problem by introducing a MaybeUninitialized library type.

union MaybeUninitialized<T> {
    value: T,
    // to solve https://github.com/rust-lang/rust/issues/36394,
    // can be implemented today with a #[lang="maybe_uninitialized"]
    buf: [u8; mem::size_of::<T>]
}

impl<T> MaybeInitialized<T> {
    fn new_uninitialized() -> Self {
        unsafe { mem::uninitialized() }
    }

    fn new(value: T) -> Self {
            MaybeInitialized { value: value }
        }

        fn get_mut(&mut self) -> *mut T {
            &mut self.value
        }

        fn get(&self) -> *const T {
            &self.value
        }
        // more APIs might be implemented
}