Dk use iodata in iso 8601 and json serialization

[dkuku]

continues on top of #14130 the changes in iso.ex are in the previous pr.
Main changes are the use of iodata in iso8601 functions for date related modules that are used in json serialization.
It also exposes the Calendar.ISO.microseconds_to_iodata(ms, p) as public because its the same logic as in the iso module that I refactored.

[josevalim]

Let’s not expose these functions from the date/time/naivedatetime/datetime APIs yet, only use it internally.

[josevalim]

@dkuku the other PR has been merged. To keep things simpler, let's only change the JSON implementation, not the actual calendar types. This will require a longer conversation about how to deal with Duration.to_iodata in json, so let's skip that in this PR as well. In a nutshell:

1. Use iodata for JSON in time, date, ndt, and datetime
2. Use microseconds_to_iodata in duration

[dkuku]

I'm a bit confused about the suggested change to the JSON implementation. When you mentioned "Only change the JSON implementation," did you mean that I should move the logic for generating the ISO 8601 representation as iodata to the JSON.Encoder module, and keep the existing to_iso8601 functions in the calendar types (e.g., Date, Time, NaiveDateTime, DateTime) unchanged? I had this in mind when I wake up but there are multiple function heads depending on the calendar used.

[josevalim]

I'm a bit confused about the suggested change to the JSON implementation. When you mentioned "Only change the JSON implementation," did you mean that I should move the logic for generating the ISO 8601 representation as iodata to the JSON.Encoder module, and keep the existing to_iso8601 functions in the calendar types (e.g., Date, Time, NaiveDateTime, DateTime) unchanged? I had this in mind when I wake up but there are multiple function heads depending on the calendar used.

Yes, exactly. Add a new clause that, if the calendar type is Calendar.ISO, we use the special path, otherwise we fallback to today's implementation.

[dkuku]

I refactored this pr. Only 2 files are changed now.

[josevalim]

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