Core: Improve iteration speed of Region.Register objects (#4583)

Without implementing __iter__ directly, calling iter() on a
Region.Register on Python 3.12 would return a new generator implemented
as follows:
```py
        def __iter__(self) -> int:
            i = 0
            try:
                while True:
                    v = self[i]
                    yield v
                    i += 1
            except IndexError:
                return None
```
This was determined by disassembling the returned generator with
dis.dis() and then constructing a function that disassembles into the
same bytecode.

The iterator returned by `iter(self._list)` is faster than this
generator, so using it slightly improves generation performance on
average.

Iteration of Region.Register objects is used a lot in
`CollectionState.update_reachable_regions` in both of the private
_update methods that get called. The performance gain here will vary
depending on how many regions a world has and how many exits those
regions have on average.

For a game like Blasphemous, with a lot of regions and exits, generation
of 10 template Blasphemous yamls with `--skip_output --seed 1` and
progression balancing disabled went from 19.0s to 16.4s (14.2% reduction
in generation duration).

BaseClasses.py

@@ -1106,6 +1106,9 @@ class Region:
         def __len__(self) -> int:
             return self._list.__len__()
 
+        def __iter__(self):
+            return iter(self._list)
+
         # This seems to not be needed, but that's a bit suspicious.
         # def __del__(self):
         #     self.clear()