Auto merge of #108273 - tspiteri:const_slice_split_at_not_mut, r=dtolnay

Stabilize const slice::split_at

This stabilizes the use of the following method in const context:

```rust
impl<T> [T] {
    pub const fn split_at(&self, mid: usize) -> (&[T], &[T]);
}
```

cc tracking issue #101158

Author: bors