Change format_any_calendar to format

[sffc]

After seeing more examples, I think we should rename DTF::format_any_calendar to format.

1. This is the function most people should be using, so we should give it a nicer name.
2. The exact behavior of which calendar converts at what time is not important for getting started. This doesn't harm program correctness in general.
3. It should be only slightly more expensive than format_same_calendar.

Note: the function has previously been called convert_and_format, but never just format.

[sffc]

@robertbastian agrees.

@Manishearth @zbraniecki thoughts?

[Manishearth]

I'm fine with this. Slight preference for the explicitness but I don't find format_any_calendar to be particularly more explicit