implement UniqueEntityArray (#17954)

# Objective

Continuation of #17589 and #16547.

`get_many` is last of the `many` methods with a missing `unique`
counterpart.
It both takes and returns arrays, thus necessitates a matching
`UniqueEntityArray` type!
Plus, some slice methods involve returning arrays, which are currently
missing from `UniqueEntitySlice`.

## Solution

Add the type, the related methods and trait impls.

Note that for this PR, we abstain from some methods/trait impls that
create `&mut UniqueEntityArray`, because it can be successfully
mem-swapped. This can potentially invalidate a larger slice, which is
the same reason we punted on some mutable slice methods in #17589. We
can follow-up on all of these together in a following PR.

The new `unique_array` module is not glob-exported, because the trait
alias `unique_array::IntoIter` would conflict with
`unique_vec::IntoIter`.
The solution for this is to make the various `unique_*` modules public,
which I intend to do in yet another PR.

Author: Victoronz

crates/bevy_ecs/src/entity/mod.rs

@@ -70,6 +70,10 @@ mod unique_slice;
 
 pub use unique_slice::*;
 
+mod unique_array;
+
+pub use unique_array::UniqueEntityArray;
+
 use crate::{
     archetype::{ArchetypeId, ArchetypeRow},
     change_detection::MaybeLocation,