message: Fix `Widget::width` implementation of `Checkbox`
diff: use std::hash::Hash;
  use crate::input::{mouse, ButtonState};
  -use crate::{layout, Element, Event, Hasher, Layout, Point, Widget};
  +use crate::{layout, Element, Event, Hasher, Layout, Length, Point, Widget};
  pub use iced_core::Checkbox;
  @@ -10,6 +10,10 @@ impl<Message, Renderer> Widget<Message, Renderer> for Checkbox<Message>
  where
  Renderer: self::Renderer,
  {
  + fn width(&self) -> Length {
  + Length::Fill
  + }
  +
  fn layout(
  &self,
  renderer: &Renderer,
  