Add __contains__ to MapKeys

I ran into this when I tried to pass `.keys()` to something declared
to take a `Collection`, and it failed because it lacked
`__contains__`. (I didn't actually care about `__contains__`, but mypy
did.)

So I came to add it to the stubs and discovered that it *was* actually
missing, and since the fallback is woefully slow I went and
implemented it.

immutables/_map.c

@@ -2810,9 +2810,23 @@ map_new_items_view(MapObject *o)
 /////////////////////////////////// _MapKeys_Type
 
 
+static int
+map_tp_contains(BaseMapObject *self, PyObject *key);
+
+static int
+_map_keys_tp_contains(MapView *self, PyObject *key)
+{
+	return map_tp_contains((BaseMapObject *)self->mv_obj, key);
+}
+
+static PySequenceMethods _MapKeys_as_sequence = {
+    .sq_contains = (objobjproc)_map_keys_tp_contains,
+};
+
 PyTypeObject _MapKeys_Type = {
     PyVarObject_HEAD_INIT(NULL, 0)
     "keys",
+    .tp_as_sequence = &_MapKeys_as_sequence,
     VIEW_TYPE_SHARED_SLOTS
 };
 

immutables/_protocols.py

@@ -32,6 +32,7 @@
 class MapKeys(Protocol[KT_co]):
     def __len__(self) -> int: ...
     def __iter__(self) -> Iterator[KT_co]: ...
+    def __contains__(self, __key: object) -> bool: ...
 
 
 class MapValues(Protocol[VT_co]):

tests/test_map.py

@@ -1395,6 +1395,10 @@ def test_kwarg_named_col(self):
         self.assertEqual(dict(self.Map(a=0, col=1)), {"a": 0, "col": 1})
         self.assertEqual(dict(self.Map({"a": 0}, col=1)), {"a": 0, "col": 1})
 
+    def test_map_keys_contains(self):
+        m = self.Map(foo="bar")
+        self.assertTrue("foo" in m.keys())
+
 
 class PyMapTest(BaseMapTest, unittest.TestCase):