Rollup merge of #54058 - Kerollmops:slice-dedup, r=shepmaster

Introduce the partition_dedup/by/by_key methods for slices

This PR propose to add three methods to the slice type, the `partition_dedup`, `partition_dedup_by` and `partition_dedup_by_key`. The two other methods are based on `slice::partition_dedup_by`.

These methods take a mutable slice, deduplicates it and moves all duplicates to the end of it, returning two mutable slices, the first containing the deduplicated elements and the second all the duplicates unordered.

```rust
let mut slice = [1, 2, 2, 3, 3, 2];

let (dedup, duplicates) = slice.partition_dedup();

assert_eq!(dedup, [1, 2, 3, 2]);
assert_eq!(duplicates, [3, 2]);
```

The benefits of adding these methods is that it is now possible to:
  - deduplicate a slice without having to allocate and possibly clone elements on the heap, really useful for embedded stuff that can't allocate for example.
  - not loose duplicate elements, because, when using `Vec::dedup`, duplicates elements are dropped. These methods add more flexibillity to the user.

Note that this is near a copy/paste of the `Vec::dedup_by` function, once this method is stable the goal is to replace the algorithm in `Vec` by the following.

```rust
pub fn Vec::dedup_by<F>(&mut self, same_bucket: F)
    where F: FnMut(&mut T, &mut T) -> bool
{
    let (dedup, _) = self.as_mut_slice().partition_dedup_by(same_bucket);
    let len = dedup.len();
    self.truncate(len);
}
```

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