Reconsider shape of DateDuration

[sffc]

In #6992, I landed a model for DateDuration that corresponds to the requirements of Temporal, specifically that it is signed, and all fields must share the same sign.

I pushed a DateDuration with public fields, including a bool is_negative. This might not be the most ergonomic design. Better might be a struct with private fields and getters that return signed values.

CC @robertbastian

[Manishearth]

We should also consider whether we should be sharing types with DurationFormat's Duration (which is a date/time duration)