Start adding concrete format functions

[sffc]

#5269 (comment)

This approach we discussed doesn't completely work because it doesn't accomplish the type inference I wanted to see, because I need to accept a Date<A: AsCalendar>, not an actual concrete Date<C>. This voids most of the benefit of having concrete functions because then you can't .into() or .parse() into the type being formatted.

Thought: if I made .format_date take a concrete Date<C> (disallowing the use of Ref), I could keep .format as-is, and then these concrete format functions become nice-to-haves that can be added on later. I think this is most aligned with the design @Manishearth had preferred anyway.

Would be nice to land this in 2.0-Alpha, but it's fine if it changes in 2.0-Beta.

CC @Manishearth @robertbastian

[sffc]

Will re-do this.