Implement compare functionality and some more traits (#82)

This adds some `compare` functions to `Date` and `DateTime`, and
Ordering for `ZonedDateTime`.

The compare functions are notably different from `Eq` and `PartialEq` as
they compare the internal ISO representation ignoring the Calendar

Author: nekevss

src/components/date.rs

@@ -31,6 +31,18 @@ pub struct Date {
     calendar: Calendar,
 }
 
+impl Ord for Date {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.iso.cmp(&other.iso)
+    }
+}
+
+impl PartialOrd for Date {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 // ==== Private API ====
 
 impl Date {

src/components/datetime.rs

@@ -28,6 +28,18 @@ pub struct DateTime {
     calendar: Calendar,
 }
 
+impl Ord for DateTime {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.iso.cmp(&other.iso)
+    }
+}
+
+impl PartialOrd for DateTime {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 // ==== Private DateTime API ====
 
 impl DateTime {

src/components/tz.rs

@@ -13,8 +13,7 @@ pub const TIME_ZONE_PROPERTIES: [&str; 3] =
     ["getOffsetNanosecondsFor", "getPossibleInstantsFor", "id"];
 
 /// A Temporal `TimeZone`.
-#[derive(Debug, Clone)]
-#[allow(unused)]
+#[derive(Debug, Clone, PartialEq, Eq, PartialOrd, Ord)]
 pub struct TimeZone {
     pub(crate) iana: Option<String>, // TODO: ICU4X IANA TimeZone support.
     pub(crate) offset: Option<i16>,

src/components/zoneddatetime.rs

@@ -12,13 +12,25 @@ use super::calendar::CalendarDateLike;
 
 /// The native Rust implementation of `Temporal.ZonedDateTime`.
 #[non_exhaustive]
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ZonedDateTime {
     instant: Instant,
     calendar: Calendar,
     tz: TimeZone,
 }
 
+impl Ord for ZonedDateTime {
+    fn cmp(&self, other: &Self) -> std::cmp::Ordering {
+        self.instant.cmp(&other.instant)
+    }
+}
+
+impl PartialOrd for ZonedDateTime {
+    fn partial_cmp(&self, other: &Self) -> Option<std::cmp::Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 // ==== Private API ====
 
 impl ZonedDateTime {