Make timezone a per-instance value (#48)

* Add constructor for timezone.

Add example to README.

* Update md.

* Make timezone a per-instance value.

Timezone and clock should be handled similarly. If clocks are switched
back to resources, then timezones could be switched back as well.

README.md

@@ -52,8 +52,7 @@ read the current time and to measure elapsed time.
 
 ### Non-goals
 
-WASI Clocks is not aiming to cover timezones, daylight savings time, date
-formatting, or modifying the time of a clock.
+WASI Clocks is not aiming to cover date formatting, or modifying the time of a clock.
 
 ### API walk-through
 
@@ -66,13 +65,11 @@ default-monotonic-clock: monotonic-clock
 ```
 
 ```rust
-   let clock: MonotonicClock = default_monotonic_clock();
-
-   let start: Instant = monotonic_clock_now(clock);
+   let start: Instant = monotonic_clock::now(clock);
 
    // some stuff
 
-   let stop: Instant = monotonic_clock_now(clock);
+   let stop: Instant = monotonic_clock::now(clock);
 
    let elapsed: Instant = stop - start;
 ```
@@ -81,13 +78,21 @@ default-monotonic-clock: monotonic-clock
 #### Telling the current human time:
 
 ```rust
-    let clock: WallClock = default_wall_clock();
-
-    let the_current_time = wall_clock_now();
+    let the_current_time = wall_clock::now();
 
     println!("it has been {} seconds and {} nanoseconds since the Unix epoch!", the_current_time.seconds, the_current_time.nanoseconds);
 ```
 
+#### Retrieving the timezone:
+
+```rust
+    let datetime: Datetime = wall_clock::now();
+
+    let timezone_display: TimezoneDisplay = timezone::display(datetime);
+
+    println!("the timezone is {}", timezone_display.name);
+```
+
 ### Detailed design discussion
 
 ### What should the type of a timestamp be?

example-world.md

@@ -149,7 +149,7 @@ also known as <a href="https://en.wikipedia.org/wiki/Unix_time">Unix Time</a>.</
 <p>
 #### <a name="timezone_display">`record timezone-display`</a>
 <p>Information useful for displaying the timezone of a specific <a href="#datetime"><code>datetime</code></a>.</p>
-<p>This information may vary within a single <a href="#timezone"><code>timezone</code></a> to reflect daylight
+<p>This information may vary within a single <code>timezone</code> to reflect daylight
 saving time adjustments.</p>
 <h5>Record Fields</h5>
 <ul>
@@ -179,13 +179,6 @@ representation of the UTC offset may be returned, such as <code>-04:00</code>.</
 should return false.</p>
 </li>
 </ul>
-<h4><a name="timezone"><code>type timezone</code></a></h4>
-<p><code>u32</code></p>
-<p>A timezone.
-<p>In timezones that recognize daylight saving time, also known as daylight
-time and summer time, the information returned from the functions varies
-over time to reflect these adjustments.</p>
-<p>This <a href="https://github.com/WebAssembly/WASI/blob/main/docs/WitInWasi.md#Resources">represents a resource</a>.</p>
 <hr />
 <h3>Functions</h3>
 <h4><a name="display"><code>display: func</code></a></h4>
@@ -197,7 +190,6 @@ daylight saving time is active.</p>
 saving time.</p>
 <h5>Params</h5>
 <ul>
-<li><a name="display.this"><code>this</code></a>: <a href="#timezone"><a href="#timezone"><code>timezone</code></a></a></li>
 <li><a name="display.when"><code>when</code></a>: <a href="#datetime"><a href="#datetime"><code>datetime</code></a></a></li>
 </ul>
 <h5>Return values</h5>
@@ -208,17 +200,9 @@ saving time.</p>
 <p>The same as <a href="#display"><code>display</code></a>, but only return the UTC offset.</p>
 <h5>Params</h5>
 <ul>
-<li><a name="utc_offset.this"><code>this</code></a>: <a href="#timezone"><a href="#timezone"><code>timezone</code></a></a></li>
 <li><a name="utc_offset.when"><code>when</code></a>: <a href="#datetime"><a href="#datetime"><code>datetime</code></a></a></li>
 </ul>
 <h5>Return values</h5>
 <ul>
 <li><a name="utc_offset.0"></a> <code>s32</code></li>
 </ul>
-<h4><a name="drop_timezone"><code>drop-timezone: func</code></a></h4>
-<p>Dispose of the specified input-stream, after which it may no longer
-be used.</p>
-<h5>Params</h5>
-<ul>
-<li><a name="drop_timezone.this"><code>this</code></a>: <a href="#timezone"><a href="#timezone"><code>timezone</code></a></a></li>
-</ul>

wit/timezone.wit

@@ -1,30 +1,17 @@
 interface timezone {
     use wall-clock.{datetime}
 
-    /// A timezone.
-    ///
-    /// In timezones that recognize daylight saving time, also known as daylight
-    /// time and summer time, the information returned from the functions varies
-    /// over time to reflect these adjustments.
-    ///
-    /// This [represents a resource](https://github.com/WebAssembly/WASI/blob/main/docs/WitInWasi.md#Resources).
-    type timezone = u32
-
     /// Return information needed to display the given `datetime`. This includes
     /// the UTC offset, the time zone name, and a flag indicating whether
     /// daylight saving time is active.
     ///
     /// If the timezone cannot be determined for the given `datetime`, return a
     /// `timezone-display` for `UTC` with a `utc-offset` of 0 and no daylight
     /// saving time.
-    display: func(this: timezone, when: datetime) -> timezone-display
+    display: func(when: datetime) -> timezone-display
 
     /// The same as `display`, but only return the UTC offset.
-    utc-offset: func(this: timezone, when: datetime) -> s32
-
-    /// Dispose of the specified input-stream, after which it may no longer
-    /// be used.
-    drop-timezone: func(this: timezone)
+    utc-offset: func(when: datetime) -> s32
 
     /// Information useful for displaying the timezone of a specific `datetime`.
     ///