Better support for weeks

[davedelong]

Desired functionality:

• given a Fixed<LTOEDay>, get the week (Array<Fixed<Day>>) that contains it
• given a Fixed<GTOEMonth>, get an array of weeks in it, with an option to pad out week arrays so they're all 7 days
• get the last day of a week

[davedelong]

Thinking about this some more, I think I could get away with an actual Week unit type; it would be GTOEDay and LTOEMonth. It would require significant verification that the current generic extensions to Fixed<Unit> apply correctly.

However, it would mean that it should get most functionality "for free".