An OrderedSet is a custom MutableSet that remembers its order, so that every
entry has an index that can be looked up.

Based on a recipe originally posted to ActiveState Recipes by Raymond Hettiger,
and released under the MIT license:

    http://code.activestate.com/recipes/576694-orderedset/

Rob Speer's changes are as follows:

    - changed the content from a doubly-linked list to a regular Python list.
      Seriously, who wants O(1) deletes but O(N) lookups by index?
    - add() returns the index of the added item
    - index() just returns the index of an item
    - added a __getstate__ and __setstate__ so it can be pickled
    - added __getitem__
    - __getitem__ and index() can be passed lists or arrays, looking up all
      the elements in them to perform NumPy-like "fancy indexing"

Tested on Python 2.6, 2.7, 3.3, 3.4, 3.5, PyPy, and PyPy3.