Make Vec::split_at_spare_mut public

This commit introduces a new method to the public API, under
`vec_split_at_spare` feature gate:

```rust
impl<T, A: Allocator> impl Vec<T, A> {
    pub fn split_at_spare_mut(&mut self) -> (&mut [T], &mut [MaybeUninit<T>]);
}
```

The method returns 2 slices, one slice references the content of the vector,
and the other references the remaining spare capacity.

The method was previously implemented while adding `Vec::extend_from_within`,
and used to implement `Vec::spare_capacity_mut` (as the later is just a
subset of former one).

0 files changed, 0 insertions, 0 deletions