Feature: parttime subtraction to produce a parital difftime

[dgkf]

To provide a more familiar interface for creating difftimes, parttime should provide:

`-`(<partial_time>, <partial_time>) -> <partial_difftime>

In all cases, a partial difftime should be the widest resulting range from two partial difftimes based on the upper and lower imputed values. This includes missing-in-the-middle partial datetimes.