Provide convenience constructors for Civil types

[spyoungtech]

Notably, Jiff does not provide a Time::now, Date::now (or Date::today), or DateTime::now constructor like it does for Timestamp or Zoned.

This may be for good reason, but ergonomically I think users could reasonably expect the following interfaces:

today := Date.today()
now := DateTime.now()
current_time := Time.now()

These should be equivalent to:

today := Zoned.now().to_date()
now := Zoned.now().to_datetime()
current_time := Zoned.now().to_time()

[BurntSushi]

This may be for good reason

Yes! Because "now" refers to an instant in a time, but civil time is not.

Jiff takes its cues from Temporal, which also lacks these sorts of constructors. Temporal has had many person-years of effort and development poured into its design by multiple domain experts.

It should generally not be that common to need something like jiff::civil::DateTime::now(). Instead, users are encouraged to use Zoned, which should provide all the functionality you would get with jiff::civil::DateTime, but with the advantage of being time zone aware and corresponding to a physical instant in time.

[spyoungtech]

Makes sense to me. I'm also inclined to drive users to favor the use of Zoned/Timestamp where possible.

Though, it does appear that Temporal.Now does allow you to directly create its "plain" objects, including those that do not represent a precise instant in time, like Temporal.Now.plainDateTimeISO(). Unless I'm misunderstanding that somehow.

Even if "now" may carry a potentially-misleading connotation of being an instant in time, I think users understand (and expect the presence of) constructors (or constructor-like static methods) like now even for naive datetime objects. I may have to think on this more, though.