Support humanizing durations

[wrouesnel]

The time support is missing support for humanizing durations into regular units: i.e. a duration of some huge number of nanoseconds should be able to be converted into "X years, 21 days, 14 hours, 31 seconds".

While there's a huge range of variability here, IMO the most common case of durations is the simple "group by the next largest unit" aspect.