Add 'buffer' and reenable _abcoll

'buffer' is currently just an empty class.

from_cpython/Lib/_abcoll.py

 # Copyright 2007 Google, Inc. All Rights Reserved.
 # Licensed to PSF under a Contributor Agreement.
 
-# Pyston change: disable using the _abcoll module for now.
-raise NotImplementedError()
-
 """Abstract Base Classes (ABCs) for collections, according to PEP 3119.
 
 DON'T USE THIS MODULE DIRECTLY!  The classes here should be imported

from_cpython/Lib/collections.py

@@ -2,8 +2,8 @@ __all__ = ['Counter', 'deque', 'defaultdict', 'namedtuple', 'OrderedDict']
 # For bootstrapping reasons, the collection ABCs are defined in _abcoll.py.
 # They should however be considered an integral part of collections.py.
 # Pyston change: disable using the _abcoll module for now.
-# from _abcoll import *
-# import _abcoll
+from _abcoll import *
+import _abcoll
 # __all__ += _abcoll.__all__
 
 from _collections import deque, defaultdict

src/runtime/builtin_modules/builtins.cpp

@@ -1138,5 +1138,8 @@ void setupBuiltins() {
     builtins_module->giveAttr("classmethod", classmethod_cls);
     builtins_module->giveAttr(
         "eval", new BoxedBuiltinFunctionOrMethod(boxRTFunction((void*)eval, UNKNOWN, 1, 0, false, false), "eval"));
+
+    BoxedClass* buffer_cls = BoxedHeapClass::create(type_cls, object_cls, NULL, 0, 0, sizeof(Box), false, "buffer");
+    builtins_module->giveAttr("buffer", buffer_cls);
 }
 }

test/tests/userlist_test.py

+import UserList