implement iterators that yield UniqueEntitySlice

[Victoronz]

Objective

Continuation of #17589 and #16547.

Slices have several methods that return iterators which themselves yield slices, which we have not yet implemented.
An example use is par_iter_many style logic.

Solution

Their implementation is rather straightforward, we simply delegate all impls to [T].
The resulting iterator types need their own wrappers in the form of UniqueEntitySliceIter and UniqueEntitySliceIterMut.

We also add three free functions that cast slices of entity slices to slices of UniqueEntitySlice.
These three should be sufficient, though infinite nesting is achievable with a trait (like TrustedEntityBorrow works over infinite reference nesting), should the need ever arise.

[alice-i-cecile]

As always, clean work. Thanks!

[chescock]

This comment was a little unclear to me at first. I wasn't sure whether this meant the slices were unique, or the contents of each slice were unique, or the contents of all slices together were unique. Since you have fn windows(), it must be the middle one.

Maybe it would help to use the UniqueEntitySlice type name? That would make the meaning of Unique unambiguous. Maybe something like "An iterator that yields &UniqueEntitySlices. Note that an entity may appear in more than one UniqueEntitySlice, but no more than once within each UniqueEntitySlice."

(And similarly for UniqueEntitySliceIterMut.)

[Victoronz]

I've clarified a bit!