extend OrderedSet with convenience methods ensure_at_front/end..

[dec1]

…l eg in implementing a list of "recent documents"

Checklist

• [x] I've looked at the contribution guidelines.
• [x] This pull request is complete and ready for review.

Description

• Bugfix: Fails to insert at index 0 into empty container (off by one error)

• For situations such as implementing a list of "recent documents" eg for text editor, it's necessary to maintain an ordered set, but with the additional capability of prepending/appending a new instance if the object doesn't yet exist in the set, or if it does, shifting the object to the front/back. This functionality has been added. Furthermore, the convenience method contains, which can be used by clients without requiring them to know the internal representation of the class, (and debug_str useful for printing/debugging)