Where it was straightforward to do so, I've updated the users to also
use ByteStrings for their file paths, but most of them have a temporary
String::from_byte_string() call instead.
The user can now save, load, and view calendars. A calendar is made up
of an array of events which are saved in a JSON file. In the future we
should implement the iCalendar standard instead of using a custom
format.