Provide access to subsecond nanos in the public interface of Duration. #154
Description
The way the Duration struct is implemented, if there are more total nanoseconds represented than i64::MAX, it is impossible to see greater than millisecond precision on the outside despite that information being stored. It would be useful to have a method similar to std::time::Duration::subsec_nanos which can return only the number of nanoseconds past the nearest second.
Additionally, for interfacing with chrono, it would be useful to have a constructor to create a Duration from a number of whole seconds plus the additional nanoseconds similar to std::time::Duration::new to efficiently construct Duration objects with full precision.