StableSet fork merge

[idanmiara]

Closes https://github.com/rspeer/ordered-set/issues/91

• Major refactor
• Added a StableSet implementation, as a base class for OrderedSet.
• Added Many functions to OrderedSet, to be more complete and more compatible with other implementations.
  • popitem(last: bool = True), similar to dict.popitem (note minor incompatibility with another implementation (orderedset) that have the last keyword in the pop function)
  • move_to_end(key), similar to dict.move_to_end
  • le, lt, ge, gt - to improve subset/superset testing
• Minimum Python version is 3.8 (because reversed)
• Fix: OrderedSet.update now raised a TypeError instead of a ValueError when the type of the input is incorrect
• Added many new tests, and all the tests from 2 other implementations.

[rspeer]

I can see why these would be desired features, but I don't want to increase the scope of this library. I'd have no ill will if you just released and packaged your own fork.