Add Option::as_slice(_mut)

This adds the following functions:

* `Option<T>::as_slice(&self) -> &[T]`
* `Option<T>::as_slice_mut(&mut self) -> &[T]`

The `as_slice` and `as_slice_mut` functions benefit from an
optimization that makes them completely branch-free.

Note that the optimization's soundness hinges on the fact that either
the niche optimization makes the offset of the `Some(_)` contents zero
or the mempory layout of `Option<T>` is equal to that of
`Option<MaybeUninit<T>>`.

library/core/src/lib.rs

@@ -134,6 +134,7 @@
 #![feature(const_option)]
 #![feature(const_option_ext)]
 #![feature(const_pin)]
+#![feature(const_pointer_byte_offsets)]
 #![feature(const_pointer_is_aligned)]
 #![feature(const_ptr_sub_ptr)]
 #![feature(const_replace)]