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.

Steps / History

• Implement the feature as a PR (Introduce the partition_dedup/by/by_key methods for slices #54058)
• Stabilization PR (see instructions on forge)

Unresolved Questions

• Should the methods only return one slice instead of a tuple with two slices? (comment)
• Annotate it with #[must_use]?
• slice::sort_by_key has more restrictions than slice::sort_by #34162