MailCraft crates/ical at a44a450: content lines, DATE/DATE-TIME values,
durations and text, the common subset of recurrence rules, and VTIMEZONE
definitions read and written, with their tests. Times are chrono values; the
zone database the writer needs is the caller's, through ZoneRules. The event
and to-do mapping onto MailCraft's model stays in MailCraft.
Co-authored-by: Martin Ball <98639962+benandez@users.noreply.github.com>
