replace Date::to_iso_week_date with Date::iso_week_date

[BurntSushi]

And also, replace ISOWeekDate::to_date with ISOWeekDate::date.

The to_ variants were deprecated in jiff 0.1.

I made this change because I felt like Zoned::iso_week_date made more sense than Zoned::to_iso_week_date, and was more consistent with Zoned::date. I then bubbled this change down to Date so that the APIs were named consistently between Zoned and Date.

Jiff arguably doesn't use the to_ prefix perfectly consistently across the crate, and is also somewhat inconsistent with the Rust API guidelines on naming. The guidelines are a little troublesome in this context because they depend, somewhat, on the "cost" of the conversion methods. Zoned::date, for example, is effectively free (it's a Copy of a type that is 4 bytes big). But Zoned::iso_week_date is not, since there is a conversion from one calendar system to another happening. Moreover, Zoned::date was not always effectively free in my initial design, but became such by the first release. In theory it could become more expensive in a future release, but probably not.

This situation repeats itself in a few other places. Basically, Jiff blurs the line between methods that do work to return a value and methods that return a value as-is. I think following the naming convention here rigorously would end up with a naming scheme that is seemingly arbitrary, where some methods have to_date while others have date and so on. And the naming could end up becoming wrong if the implementation details change.

One downside here though is that one cannot use the naming to get a sense of which methods are "cheap" and which are "not cheap." I think probably in most cases it won't matter, and folks will just need to rely on profiling to determine when a method call should be amortized.