[issue16991] Add OrderedDict written in C

[Eric Snow]

Eric Snow added the comment:

The concern about reference cycles here lies in their existence in the linked-list.  To see what I mean, check out the use of weakrefs in the pure Python implementation at Lib/collections/__init__.py.  As the current implementation does not use PyObjects for the linked-list, I'm going to call this a non-issue.

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<http://bugs.python.org/issue16991>
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