[capi] Add ZonedDateTime::{equals,offset} (#362)

Author: Manishearth

temporal_capi/bindings/c/ZonedDateTime.h

@@ -200,6 +200,17 @@ pub mod ffi {
             self.0.compare_instant(&other.0)
         }
 
+        pub fn equals(&self, other: &Self) -> bool {
+            self.0 == other.0
+        }
+
+        pub fn offset(&self, write: &mut DiplomatWrite) -> Result<(), TemporalError> {
+            let string = self.0.offset()?;
+            // throw away the error, this should always succeed
+            let _ = write.write_str(&string);
+            Ok(())
+        }
+
         pub fn start_of_day(&self) -> Result<Box<ZonedDateTime>, TemporalError> {
             self.0
                 .start_of_day()