Full audit of datetime renames

[sffc]

For 2.0 Beta:

• NeoSkeletonLength should be Length
• There were some questions about the name of IsAnyCalendarKind
• There should be a format_iso function on DateTimeFormatter, possibly replacing convert_and_format
• strict_format should maybe be format_same_calendar
• FormattedNeoDateTime should be FormattedDateTime
• DateTimePatternFormatter should have a single generic format function
• DateTimePatternFormatter and friends should use FSet instead of R
• DateTimeNames::with_pattern should be try_with_pattern and with_pattern_unchecked (Consider exact policy around DateTimeError in datetime formatting #4336 (comment))

For 2.0 Final:

• The markers should be renamed. They mostly still have "neo" in the name and/or the module

[Manishearth]

For 2.0 Final I would like every function involving fieldsets to have docs pointing to the fieldset module. Plus every trait like AllAnyCalendarFormattingDataMarkers should explain what its common bounds mean i nrelation to fieldsets, and what to do when the trait isn't implemented.

[Manishearth]

(I'm in favor of all bullet points here. Replacing convert_and_format seems fine, we only have it because that was the initial behavior of the module and we wanted to keep it)

[sffc]

There were some questions about the name of IsAnyCalendarKind

Did you want to weigh in on what this trait should be called? I had recorded a comment that someone didn't like the name but didn't have an alternative. I'm personally still happy with the name.

[Manishearth]

No strong opinions, fine with the name. IsConcreteCalendar perhaps.

[sffc]

No strong opinions, fine with the name. IsConcreteCalendar perhaps.

I'd kinda like to move it to icu_calendar since it is in the same realm of functionality as IntoAnyCalendar. Can you suggest an appropriate trait signature?

[Manishearth]

I'd say the same one?

One thing to note is that Calendar already has this functionality, what this trait gets us is rather datetime-specific: "if you care about a calendar, is this the right calendar, or no?" I don't see that as particularly useful for calendar consumers.