Rename rest to leftover

The length kind that asks for a part of what is left once the fixed
lengths are taken is called leftover: Len::leftover(2), Len::LEFTOVER,
Size::LEFTOVER, Len::leftover the field, and apply_leftover. It says
what it is where "rest" reads as "the remainder of the list" as often as
"the remaining space", and every agent who has touched this has reached
for a third word for it.

Locals called rest that meant a region or a widget are renamed with it,
since the word now names something else.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/widget/rect.rs

@@ -35,11 +35,11 @@ impl Widget for Rect {
             thickness: self.thickness,
             inner_radius: self.inner_radius,
         });
-        Size::REST
+        Size::LEFTOVER
     }
 
     fn size_hint(&self, _: Axis) -> Option<Len> {
-        Some(Len::REST)
+        Some(Len::LEFTOVER)
     }
 
     /// Its box is its primitive's own region, so a new one is written there.