'insert', 'swap', 'get_by_index' for `OrderedDict`

[dg-pb]

@blhsing

If your application needs a mapping that can efficiently access items by indices, then SortedDict , whose keys are maintained in a bisect-managed list, would be your best choice, with which you can obtain a key-item pair by an index through its indexable items() view.

This is what I am aiming to improve upon. Not necessarily in performance, but at least in implementing it more concisely/elegantly.