rename MaybeUninit slice methods
The `first` methods conceptually point to the whole slice, not just its first element, so rename them to be consistent with the raw ptr methods on ref-slices.
Also, do the equivalent of https://github.com/rust-lang/rust/pull/76047 for the slice reference getters, and make them part of https://github.com/rust-lang/rust/issues/63569 (so far they somehow had no tracking issue).
* first_ptr -> slice_as_ptr
* first_ptr_mut -> slice_as_mut_ptr
* slice_get_ref -> slice_assume_init_ref
* slice_get_mut -> slice_assume_init_mut
cdc8f0606d