Duration struct

[younies]

Hi @sffc

@DesmondWillowbrook and @younies think we should create a struct called Duration to set the duration values for the input to the DurationFormatter. Or there is another Duration object that we can use.

[sffc]

To get started, I think duration should define its own struct with the 9 fields required by Intl.DurationFormat, probably each of type u64, which is a bit big but it's fine for a short-lived object. Eventually we might want to add a trait or conversions into other Duration types. We already have 2 other duration types in ICU4X, one in the ixdtf crate and another in the icu_calendar crate, but they are both very specific to their use cases.

CC @Manishearth

[sffc]

CC @jedel1043