Support LOAD_METHOD and CALL_METHOD from Python 3.7+

Fixes #163

tests/input/load_method.py

@@ -0,0 +1,15 @@
+class C:
+    def test1(self):
+        a = 1
+
+    def test2(self, x, y, z):
+        a = x * y + z
+
+    # @staticmethod -- TODO: Fix decorators
+    def testS():
+        a = 3
+
+c = C()
+c.test1()
+c.test2(42, 5, -1)
+C.testS()