Put place before the rel base, and say what a Place decides

An argument that is usually `None` goes last. So `widget_at` and `place_at`
take the place first, and `narrow_rel_base` after it.

`Place` and `Part` also now say which of the two boxes they decide, since
that is the question a caller has to answer to pick between them: `Part` is
the child's region, said as a part of the caller's own, and `Within`/`Fill`
is what becomes of the placement in it.

tests/cases/retained.rs

@@ -216,8 +216,8 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            [None; 2],
             [Place::Within(Part::WHOLE), Place::Within(Part::From(top))],
+            None,
         );
         Size::LEFTOVER
     }
@@ -877,11 +877,11 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Within(Part::From(self.region.x)),
                     Place::Within(Part::From(self.region.y)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -966,11 +966,11 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [Some(self.frame.x.len()), None],
                 [
                     Place::Fill(Part::From(self.region.x)),
                     Place::Fill(Part::From(self.region.y)),
                 ],
+                [Some(self.frame.x.len()), None],
             );
             Size::LEFTOVER
         }
@@ -1136,11 +1136,11 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Fill(Part::From(self.region.x)),
                     Place::Fill(Part::From(self.region.y)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -1224,7 +1224,6 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [None; 2],
                 [
                     Place::Fill(Part::From(UiSpan::new(
                         Len::px(self.start),
@@ -1232,6 +1231,7 @@ fn moving_a_childs_region_preserves_the_slot_chosen_from_its_measurement() {
                     ))),
                     Place::Fill(Part::From(UiSpan::FULL)),
                 ],
+                None,
             );
             Size::LEFTOVER
         }
@@ -1266,11 +1266,11 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    [None; 2],
                     [
                         Place::Within(Part::From(self.region.x)),
                         Place::Within(Part::From(self.region.y)),
                     ],
+                    None,
                 )
                 .size()
         }
@@ -1286,11 +1286,11 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [None; 2],
                         [
                             Place::Fill(Part::From(self.region.x)),
                             Place::Fill(Part::From(self.region.y)),
                         ],
+                        None,
                     )
                     .size(),
             );