Add ordered_get() to HistoryBuffer

[schultetwin1]

This allows users to get access to the elements of the history buffer in historical order.

[reitermarkus]

Okay, after refactoring this to self.oldest_ordered().nth(idx), I am not convinced we should have a separate method for this. Most often you will need recent() or oldest(), for the other times I think oldest_ordered().nth(n) is actually more readable and easy enough to use.

[schultetwin1]

I did not realize I could just use self.oldest_ordered().nth(idx), I agree that's fine. Thank you for all the time you spent looking at this!