perf: reduce memory allocations

This also changes `widget::column::with_children` and
`widget::row::with_children` to take an `impl IntoIterator` instead
of a `Vec`, like the `iced` variants of these functions do.

This shouldn't be a breaking change since passing in a `Vec` will still
compile and function exactly as before.

(Using `iced::widget::Column::from_vec` or
`iced::widget::Row::from_vec` isn't possible, since the elements of the
`Vec` aren't checked, so the size of the resulting `Column` or `Row`
won't adapt to the size of its children. Perhaps a new function could
be added to mirror `iced`'s?)

src/widget/calendar.rs

@@ -179,8 +179,8 @@ where
             ));
         }
 
-        let content_list = column::with_children(vec![
-            row::with_children(vec![
+        let content_list = column::with_children([
+            row::with_children([
                 date.into(),
                 crate::widget::Space::with_width(Length::Fill).into(),
                 month_controls.into(),