Refactor timezone-display into separate methods

If the timezone-display ID is an IANA ID, and we are going with the
approach of not making the localized ("PST" vs "PDT" vs "PT") name part of
this component, then the current time zone doesn't depend on the current
time.

After removing the isDST flag, timezone-display contains two pieces of
data, the ID and the UTC offset. The UTC offset is already available via
a function that takes an Instant as input. The ID could just be available
via its own function that doesn't take any input.

In that case there would be no need for timezone-display.

Author: ptomato

README.md

@@ -93,10 +93,9 @@ default-monotonic-clock: monotonic-clock
 
 ```rust
     let instant: Instant = system_clock::now();
-
-    let timezone_display: TimezoneDisplay = timezone::display(instant);
-
-    println!("the timezone is {}", timezone_display.id);
+    let id = timezone::id();
+    let offset_h = timezone::utc_offset(instant) as f64 / 3600e9;
+    println!("the timezone is {} at UTC{:+}", id, offset_h);
 ```
 
 ### Detailed design discussion

wit-0.3.0-draft/timezone.wit

@@ -5,45 +5,31 @@ interface timezone {
     @unstable(feature = clocks-timezone)
     use system-clock.{instant};
 
-    /// Return information needed to display the given `instant` in the
-    /// currently configured time zone. This includes the UTC offset and the
-    /// time zone name.
+    /// Return the IANA identifier of the currently configured timezone. The id
+    /// `UTC` indicates Coordinated Universal Time. Otherwise, this should be an
+    /// identifier from the IANA Time Zone Database.
     ///
-    /// If the currently configured timezone cannot be determined, return a
-    /// `timezone-display` for `UTC` with a `utc-offset` of 0.
+    /// For displaying to a user, the identifier should be converted into a
+    /// localized name by means of an internationalization API.
+    ///
+    /// In implementations that do not expose an actual time zone, this
+    /// should be the string `UTC`.
+    ///
+    /// In time zones that do not have an applicable name, a formatted
+    /// representation of the UTC offset may be returned, such as `-04:00`.
     @unstable(feature = clocks-timezone)
-    display: func(when: instant) -> timezone-display;
+    id: func() -> string;
 
-    /// The same as `display`, but only return the UTC offset.
+    /// The number of nanoseconds difference between UTC time and the local
+    /// time of the currently configured timezone at the exact time of
+    /// `instant`.
+    ///
+    /// The magnitude of the returned value will always be less than
+    /// 86,400,000,000,000 which is the number of nanoseconds in a day
+    /// (24*60*60*1e9).
+    ///
+    /// In implementations that do not expose an actual time zone, this
+    /// should return 0.
     @unstable(feature = clocks-timezone)
     utc-offset: func(when: instant) -> s64;
-
-    /// Information useful for displaying a specific `instant` in the currently
-    /// configured time zone.
-    @unstable(feature = clocks-timezone)
-    record timezone-display {
-        /// The number of nanoseconds difference between UTC time and the local
-        /// time of the timezone.
-        ///
-        /// The returned value will always be less than 86,400,000,000,000 which
-        /// is the number of nanoseconds in a day (24*60*60*1e9).
-        ///
-        /// In implementations that do not expose an actual time zone, this
-        /// should return 0.
-        utc-offset: s64,
-
-        /// The IANA identifier of the timezone. The id `UTC` indicates
-        /// Coordinated Universal Time. Otherwise, this should be an identifier
-        /// from the IANA Time Zone Database.
-        ///
-        /// For displaying to a user, the identifier should be converted into a
-        /// localized name by means of an internationalization API.
-        ///
-        /// In implementations that do not expose an actual time zone, this
-        /// should be the string `UTC`.
-        ///
-        /// In time zones that do not have an applicable name, a formatted
-        /// representation of the UTC offset may be returned, such as `-04:00`.
-        id: string,
-    }
 }