Choose more descriptive field names for ReserveEntitiesIterator (#15168)

No hard feelings if you don't want to make this change. This is just
something I stumbled over in my very first read of the `bevy_ecs` crate.

# Objective

- the general goal here is to improve DX slightly
- make the code easier to read in general. The previous names make the
code harder to read, especially since they are so similar.

## Solution

- choose more specific names for the fields
- `index_iter` -> `freelist_indices` : "freelist" is a well established
term in the rest of the docs in this module, so we might want to reuse
it
- `index_range` -> `new_indices` : Nothing besides the doc comment
stated that these indices were actually new/fresh

## Testing

Note that the fields are private so that this is no breaking change.
They are also only used in this one module.

crates/bevy_ecs/src/entity/mod.rs

@@ -450,26 +450,26 @@ pub struct ReserveEntitiesIterator<'a> {
     meta: &'a [EntityMeta],
 
     // Reserved indices formerly in the freelist to hand out.
-    index_iter: std::slice::Iter<'a, u32>,
+    freelist_indices: std::slice::Iter<'a, u32>,
 
     // New Entity indices to hand out, outside the range of meta.len().
-    index_range: std::ops::Range<u32>,
+    new_indices: std::ops::Range<u32>,
 }
 
 impl<'a> Iterator for ReserveEntitiesIterator<'a> {
     type Item = Entity;
 
     fn next(&mut self) -> Option<Self::Item> {
-        self.index_iter
+        self.freelist_indices
             .next()
             .map(|&index| {
                 Entity::from_raw_and_generation(index, self.meta[index as usize].generation)
             })
-            .or_else(|| self.index_range.next().map(Entity::from_raw))
+            .or_else(|| self.new_indices.next().map(Entity::from_raw))
     }
 
     fn size_hint(&self) -> (usize, Option<usize>) {
-        let len = self.index_iter.len() + self.index_range.len();
+        let len = self.freelist_indices.len() + self.new_indices.len();
         (len, Some(len))
     }
 }
@@ -589,8 +589,8 @@ impl Entities {
 
         ReserveEntitiesIterator {
             meta: &self.meta[..],
-            index_iter: self.pending[freelist_range].iter(),
+            freelist_indices: self.pending[freelist_range].iter(),
-            index_range: new_id_start..new_id_end,
+            new_indices: new_id_start..new_id_end,
         }
     }