Ordered Set implementation in Cython.

 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.