Add more indexed methods to entries

[cuviper]

For IndexedEntry, OccupiedEntry, and RawOccupiedEntryMut, this
adds move_index and swap_indices methods that work like the
top-level IndexMap methods using the current index of the entry.

For VacantEntry this adds shift_insert, while RawVacantEntryMut
adds shift_insert and shift_insert_hashed_nocheck, offering a way to
insert at a particular index while shifting other entries as needed.

Finally, for IndexMap and IndexSet this adds shift_insert using
OccupiedEntry::move_index or VacantEntry::shift_insert.