feat: function downcasting

FIXME: This downcast is unsound, function inputs, output should be checked at runtime

crates/erg_compiler/lib/std/_erg_contains_operator.py

@@ -5,6 +5,7 @@ from _erg_type import _isinstance
 from _erg_type import UnionType
 
 from collections import namedtuple
+from typing import Callable
 
 # (elem in y) == contains_operator(y, elem)
 def contains_operator(y, elem) -> bool:
@@ -62,5 +63,8 @@ def contains_operator(y, elem) -> bool:
     elif _isinstance(elem, list):
         from _erg_array import Array
         return contains_operator(y, Array(elem))
+    elif _isinstance(elem, Callable):
+        # TODO:
+        return _isinstance(y, Callable)
     else:
         return elem in y

crates/erg_compiler/lib/std/_erg_type.py

@@ -1,3 +1,5 @@
+from typing import Callable
+
 try:
     from typing import Union
 except ImportError:
@@ -43,7 +45,8 @@ def _isinstance(obj, classinfo) -> bool:
             return any(_isinstance(obj, t) for t in classinfo.__args__)
         else:
             return isinstance(obj, classinfo.__origin__)
-    elif is_type(classinfo):
-        return isinstance(obj, classinfo)
     else:
-        return False
+        try:
+            return isinstance(obj, classinfo)
+        except:
+            return False