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/dialog.rs

@@ -158,8 +158,7 @@ impl<'a, Message: Clone + 'static> From<Dialog<'a, Message>> for Element<'a, Mes
         }
 
         let mut container = widget::container(
-            widget::column::with_children(vec![content_row.into(), button_row.into()])
-                .spacing(space_l),
+            widget::column::with_children([content_row.into(), button_row.into()]).spacing(space_l),
         )
         .class(style::Container::Dialog)
         .padding(space_m)