First class support for the duration object

[cdelst]

It seems to me (forgive me if I'm wrong, I've done my best to take a look at open and closed issues, as well as documentation) that the duration object in the package doesn't have the best support around it. Comparing them, doing arithmetic on them, etc. seems to not exist at the moment. This is a little confusing to me, given the awesome simplicity of the duration rep.

Is that something there is appetite for? I would love to add a comparison function for example, among other utilities to make the duration object an esteemed citizen in this library.