Switch FFI calendar APIs over to using AnyCalendarKind for input (#324)

We clone these calendars anyway; the current API forces clients to
retain some allocated Calendar objects for no real reason. Let's
standardize around using kinds.

This breaks the CAPI, but I'm not using this stuff yet. If I decide to
start using this before a release, I'll add some compat layer for the
APIs I care about.

Author: Manishearth

src/builtins/core/calendar.rs

@@ -178,6 +178,12 @@ impl Calendar {
         matches!(self.0 .0, AnyCalendar::Iso(_))
     }
 
+    /// Returns the kind of this calendar
+    #[inline]
+    pub fn kind(&self) -> AnyCalendarKind {
+        self.0 .0.kind()
+    }
+
     /// `CalendarDateFromFields`
     pub fn date_from_partial(
         &self,