add a SETUP_WITH opcode

It speeds up the with statement and correctly looks up the special
methods involved.
diff --git a/Lib/test/test_descr.py b/Lib/test/test_descr.py
index c7989b1..627f8aa 100644
--- a/Lib/test/test_descr.py
+++ b/Lib/test/test_descr.py
@@ -1689,6 +1689,7 @@
             return isinstance(int, obj)
         def do_issubclass(obj):
             return issubclass(int, obj)
+        def swallow(*args): pass
 
         # It would be nice to have every special method tested here, but I'm
         # only listing the ones I can remember outside of typeobject.c, since it
@@ -1702,11 +1703,8 @@
             ("__instancecheck__", do_isinstance, return_true, set(), {}),
             ("__subclasscheck__", do_issubclass, return_true,
              set(("__bases__",)), {}),
-            # These two fail because the compiler generates LOAD_ATTR to look
-            # them up.  We'd have to add a new opcode to fix this, and it's
-            # probably not worth it.
-            # ("__enter__", run_context, iden),
-            # ("__exit__", run_context, iden),
+            ("__enter__", run_context, iden, set(), {"__exit__" : swallow}),
+            ("__exit__", run_context, swallow, set(), {"__enter__" : iden}),
             ]
 
         class Checker(object):