Allow DateTime objects to be serialized and deserialized

Summary:
Ref T10747. I have to move CalendarEvents to use these for everything because ICS files can have events which we can't represent with an epoch (e.g., floating "4PM" events).

Also improve some timezone/floating logic when rendering to ISO8601.

Test Plan: arc unit; see next change.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10747

Differential Revision: https://secure.phabricator.com/D16653