change `Display` implementation for `Duration`

[Thomasdezeeuw]

The current implementation doesn't really display smalls durations very well, for example PT0.000013825S is less then a millisecond, but that is not really easy to see. Coming from Go I really like it's Duration string implementation (click the function for source).

Go uses the following format: 72h3m0.5s. For durations smaller then a second it has special cases, e.g. 1.397068ms and even 426ns.

Any opinions on changing the implementation?

[quodlibetor]

A couple things:

(a) Duration is actually part of rust-lang-deprecated/time, so we can't trivially edit it. @lifthrasiir is working on a chrono-internal TimeSpan which will give more flexibility.

(b) The current display has the advantage of being ISO-8601 conformant, and so easily parsable in almost any language. If we were going to do something more human-friendly (which I'd support) my first inclination is that it would be better to put it behind a to_human_str() or display_human() (method name needs a better color) method.

What do you think?

[Thomasdezeeuw]

Is there an open issue for the work done on TimeSpan? It makes sense to accompany that change with this one. I agree with point b, having a display for humans sounds good.