Refactor Entry with more separation from the raw part
Add a core method to support set's replace_full
This lets us remove the saved key from map::OccupiedEntry. That also shrinks Entry because there's now room for the enum tag in a layout niche, although such layout is not guaranteed.
Entrywith more separation from therawpartreplace_fullmap::OccupiedEntry. That also shrinksEntrybecause there's now room for the enum tag in a layout niche, although such layout is not guaranteed.