Support arrays ([impl ViewSequence; N]) as ViewSequences

Adds the blanket impl
`impl<const N: usize, VT: ViewSequence> ViewSequence for [VT; N]`

Allows for example something like this:

```rust
fn my_tab(impl View) -> impl View {..}
h_stack(["Tab 1", "Tab 2", "Tab 3"].map(my_tab))
```

crates/xilem_core/src/sequence.rs

@@ -283,6 +283,51 @@ macro_rules! generate_viewsequence_trait {
             }
         }
 
+        impl<T, A, const N: usize, VT: $viewseq<T, A>> $viewseq<T, A> for [VT; N] {
+            type State = [VT::State; N];
+
+            fn build(&self, cx: &mut Cx, elements: &mut Vec<$pod>) -> Self::State {
+                self.each_ref().map(|vs| vs.build(cx, elements))
+            }
+
+            fn rebuild(
+                &self,
+                cx: &mut $cx,
+                prev: &Self,
+                state: &mut Self::State,
+                elements: &mut $crate::VecSplice<$pod>,
+            ) -> $changeflags {
+                self.iter()
+                    .enumerate()
+                    .map(|(i, vs)| vs.rebuild(cx, &prev[i], &mut state[i], elements))
+                    .fold(ChangeFlags::empty(), |changes_acc, changes| {
+                        changes_acc | changes
+                    })
+            }
+
+            fn message(
+                &self,
+                id_path: &[Id],
+                state: &mut Self::State,
+                message: Box<dyn std::any::Any>,
+                app_state: &mut T,
+            ) -> $crate::MessageResult<A> {
+                let mut result = $crate::MessageResult::Stale(message);
+                for (child, child_state) in self.iter().zip(state) {
+                    if let $crate::MessageResult::Stale(message) = result {
+                        result = child.message(id_path, child_state, message, app_state);
+                    } else {
+                        break;
+                    }
+                }
+                result
+            }
+
+            fn count(&self, _state: &Self::State) -> usize {
+                N
+            }
+        }
+
         /// This trait marks a type a
         #[doc = concat!(stringify!($view), ".")]
         ///

examples/hello.rs

@@ -16,7 +16,7 @@ fn app_logic(data: &mut AppData) -> impl View<AppData> {
             println!("clicked");
             data.count += 1;
         }),
-        h_stack((
+        h_stack([
             button("decrease", |data: &mut AppData| {
                 println!("clicked decrease");
                 data.count -= 1;
@@ -25,10 +25,10 @@ fn app_logic(data: &mut AppData) -> impl View<AppData> {
                 println!("clicked reset");
                 data.count = 0;
             }),
-            switch(data.is_on, |data: &mut AppData, value: bool| {
-                data.is_on = value
-            }),
-        )),
+        ]),
+        switch(data.is_on, |data: &mut AppData, value: bool| {
+            data.is_on = value
+        }),
     ))
     .with_spacing(20.0)
 }