Implement framework for class-defined built-in operators.

Now working for class-defined methods: __getitem__, __setitem__,
__add__, __sub__.  Easy to add others.

tests/basics/tests/class_item.py

@@ -0,0 +1,13 @@
+# test class with __getitem__ and __setitem__ methods
+
+class C:
+    def __getitem__(self, item):
+        print('get', item)
+        return 'item'
+
+    def __setitem__(self, item, value):
+        print('set', item, value)
+
+c = C()
+print(c[1])
+c[1] = 2

tests/basics/tests/class_number.py

@@ -0,0 +1,15 @@
+# test class with __add__ and __sub__ methods
+
+class C:
+    def __init__(self, value):
+        self.value = value
+
+    def __add__(self, rhs):
+        print(self.value, '+', rhs)
+
+    def __sub__(self, rhs):
+        print(self.value, '-', rhs)
+
+c = C(0)
+c + 1
+c - 2