Orderly-set

- `__getitem__` accepts NumPy arrays of indices, and returns a list of elements with those indices.
- Updated in-place operations that took O(N^2) time, such as .difference_update(), to take O(N) time.
- Clarified whether various methods mutate or copy the OrderedSet.
- Added `OrderedSet.get_loc` and `OrderedSet.get_indexer` as aliases for `OrderedSet.index`, for interoperability with `pandas.Index`.
- Added type stubs in a .pyi file.

- Implemented the abstract base classes `collections.MutableSet` and `collections.Sequence`.
- Changed the behavior of some methods to follow the MutableSet API.
- Indexing an OrderedSet with `[:]` returns a copy, not the same object.

- Tuples are allowable values in the set, and are not treated as "fancy indexing".
- Added `update` and `pop` methods.

- Added `discard` and `clear` methods.

- Added support for pickling.

- First Python 3 support.