Check if the specified date is a holiday. If the specified country or year is
not available, it will return Err(Error). If the date is not a holiday, it
will return Ok(false). Otherwise, it will return Ok(true).
Get holiday by ISO 3166-1 alpha-2 country code and date. If the specified country or year is
not available, it will return Err(Error). If the specified date is not a holiday, it
will return Ok(None). Otherwise, it will return Ok(Some(Holiday)).
Initialize holiday database for all the supported countries and years.
Note that this will use quite lots of memory. Please consider using
Builder to specify countries and years to load for if you’re concerned
about memory usage.