Add MaybeUninit::(slice_)as_bytes(_mut)
This adds methods to convert between `MaybeUninit<T>` and a slice of `MaybeUninit<u8>`. This is safe since `MaybeUninit<u8>` can correctly handle padding bytes in any `T`.
These methods are added:
```rust
impl<T> MaybeUninit<T> {
pub fn as_bytes(&self) -> &[MaybeUninit<u8>];
pub fn as_bytes_mut(&mut self) -> &mut [MaybeUninit<u8>];
pub fn slice_as_bytes(this: &[MaybeUninit<T>]) -> &[MaybeUninit<u8>];
pub fn slice_as_bytes_mut(this: &mut [MaybeUninit<T>]) -> &mut [MaybeUninit<u8>];
}
```
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