For reasoning, see https://github.com/rust-lang/rust/issues/63567#issuecomment-858640987

And also add backticks around `MaybeUninit`.

* Undo fn -> const fn for some fns.
* Split feature gate.
* Made all three intrinsics const

Looks like it is no longer necessary, as uninit_array() can be used
instead in the few cases where it was needed.

ManuallyDrop's documentation tells the user to use MaybeUninit instead
when handling uninitialized data. However, the main functionality of
ManuallyDrop (drop) was not available directly on MaybeUninit. Adding it
makes it easier to switch from one to the other.

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