Overload isvalid, etc. for ZonedDateTimes

[glennmoy]

Since LaxZonedDateTime extends/generalises the functionality of the Dates and TimeZones interface, it would be nice if the methods defined in this package were backwards-compatible, i.e. if we could overload isvalid, isinvalid, isambiguous, isnonexistent for DateTimes and ZonedDateTimes.

This is useful in cases where ZonedDateTimes/LaxZonedDateTimes are used interchangeably but the is conditional behavoiur whether the time variable given isvalid or not.