refactor: speed up group_by, by using deque to avoid explicit for loop

Author: zq46

src/superstream/stream.py

@@ -57,8 +57,9 @@ def sum(self, start: T = 0) -> T:
 
     def group_by(self, classifier: Callable[[T], K]) -> Dict[K, List[T]]:
         groups = {}
-        for i in self._stream:
+        def _classify(i):
             groups.setdefault(classifier(i), []).append(i)
+        deque(map(_classify, self._stream), maxlen=0)
         return groups
 
     def reduce(self, func: Callable[[T, T], T], initial: T = _initial_missing) -> Optional[T]: