An Ordered Set implementation in Cython.

Features:

* Works like a regular set, but remembers insertion order; * Is approximately 5 times faster than the pure Python implementation overall (and 5 times slower than `set`); * Compatible with Python 2.7 through 3.8; * Supports the full set interface; * Supports some list methods, like `index` and `__getitem__`. * Supports set methods against iterables.