Remove a ptr-to-int cast in CommandQueue::apply (#10475)

# Objective

- `CommandQueue::apply` calculates the address of the end of the
internal buffer as a `usize` rather than as a pointer, requiring two
casts of `cursor` to `usize`. Casting pointers to integers is generally
discouraged and may also prevent optimizations. It's also unnecessary
here.

## Solution

- Calculate the end address as a pointer rather than a `usize`.

Small note:

A trivial translation of the old code to use pointers would have
computed `end_addr` as `cursor.add(self.bytes.len())`, which is not
wrong but is an additional `unsafe` operation that also needs to be
properly documented and proven correct. However this operation is
already implemented in the form of the safe `as_mut_ptr_range`, so I
just used that.

crates/bevy_ecs/src/system/commands/command_queue.rs

@@ -97,18 +97,18 @@ impl CommandQueue {
         // flush the previously queued entities
         world.flush();
 
-        // Pointer that will iterate over the entries of the buffer.
-        let mut cursor = self.bytes.as_mut_ptr();
+        // The range of pointers of the filled portion of `self.bytes`.
+        let bytes_range = self.bytes.as_mut_ptr_range();
 
-        // The address of the end of the buffer.
-        let end_addr = cursor as usize + self.bytes.len();
+        // Pointer that will iterate over the entries of the buffer.
+        let mut cursor = bytes_range.start;
 
         // Reset the buffer, so it can be reused after this function ends.
         // In the loop below, ownership of each command will be transferred into user code.
         // SAFETY: `set_len(0)` is always valid.
         unsafe { self.bytes.set_len(0) };
 
-        while (cursor as usize) < end_addr {
+        while cursor < bytes_range.end {
             // SAFETY: The cursor is either at the start of the buffer, or just after the previous command.
             // Since we know that the cursor is in bounds, it must point to the start of a new command.
             let meta = unsafe { cursor.cast::<CommandMeta>().read_unaligned() };