Investigate integration with some components from icu4x project

[esheppa]

It would need to first be determined which project sits lower in the stack, and it may be better for any integration to be done as a separate crate

My initial thoughts of what could be useful are:

• icu_calendar - this could potentially be leveraged to implement alternative calendar systems for NaiveDate
• icu_timezone
• icu_datetime - could be an alternative to pure-rust-locales which we currently use

[djc]

An alternative to datetime would be interesting since pure-rust-locales seems to be more or less a one-off which was built for chrono but isn't getting much maintenance. Otherwise, it seems mostly undesirable to implement primitives in chrono based on other crates?