wrap EntityIndexMap/Set slices as well (#18134)

# Objective

Continuation of #17449.

#17449 implemented the wrapper types around `IndexMap`/`Set` and co.,
however punted on the slice types.
They are needed to support creating `EntitySetIterator`s from their
slices, not just the base maps and sets.

## Solution

Add the wrappers, in the same vein as #17449 and #17589 before.

The `Index`/`IndexMut` implementations take up a lot of space, however
they cannot be merged because we'd then get overlaps.

They are simply named `Slice` to match the `indexmap` naming scheme, but
this means they cannot be differentiated properly until their modules
are made public, which is already a follow-up mentioned in #17954.

Author: Victoronz