Truncating Duration

Classes like `LocalTime` and `Instant` have a method `truncatedTo(TemporalUnit unit)` that returns an instance with smaller units set to zero. A similar method should be added to `Duration`. Without this method, the user is forced to perform maths on the epoch-second or nano-of-second value, which is what java.time seeks to avoid.

The specification and implementation would be very similar to that on `Instant`.