One argument says where a child goes and what its fractions are of

`Place` was a product written as a sum -- a `Part` and a fill flag -- and
`Part` named three operations the geometry already had, under words that did
not match them. `Of` was `UiSpan::within`, `From` was `UiSpan::shift`, and
`Sized` was `placement`'s own body with the length given rather than
reported. Both enums are gone.

`PlaceDescAxis` says one axis, named after the operation it performs:
`within`, `shifted`, `sized`, and `WHOLE`. What is optional is a builder --
`fills` and `rel_base` -- so a caller writes only what it decided, and the
rel base it does not write follows the constructor: a span composed into the
caller's box narrows it, a span along a cursor does not, a decided length is
it. That was the one rule a caller could get wrong with nothing failing.

`PlaceDesc` says both axes with named fields, so `axis`, `axis_mut` and
`from_axis` work the way they do on every other pair here, and the joint
work -- resolving a region, reading the fill flags -- is written once rather
than per axis. `widget_at` and `place_at` take `impl Into<PlaceDesc>`, so a
wrapper passes a `UiRegion` and says nothing else. `widget_within` and
`ActiveData::narrow_rel_base` are deleted; `asked` and `placed` carry the
rel base their ask stated.

Cold layout is byte-identical to `84dad21`.

tests/cases/retained.rs

@@ -216,8 +216,7 @@ impl Widget for FromHint {
         let top = UiSpan::new(Len::ZERO, Len::from_parts(Rel::ZERO, len.px));
         painter.widget_at(
             &self.inner,
-            [Place::Within(Part::WHOLE), Place::Within(Part::From(top))],
-            None,
+            PlaceDesc::new(PlaceDescAxis::WHOLE, PlaceDescAxis::shifted(top)),
         );
         Size::LEFTOVER
     }
@@ -877,11 +876,10 @@ fn resizing_a_fixed_frame_recomposes_its_contents_without_drawing_them() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [
-                    Place::Within(Part::From(self.region.x)),
-                    Place::Within(Part::From(self.region.y)),
-                ],
-                None,
+                PlaceDesc::new(
+                    PlaceDescAxis::shifted(self.region.x),
+                    PlaceDescAxis::shifted(self.region.y),
+                ),
             );
             Size::LEFTOVER
         }
@@ -966,11 +964,11 @@ fn glyph_origins_compose_identically_when_drawn_and_when_retained() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [
-                    Place::Fill(Part::From(self.region.x)),
-                    Place::Fill(Part::From(self.region.y)),
-                ],
-                [Some(self.frame.x.len()), None],
+                PlaceDesc::new(
+                    PlaceDescAxis::shifted(self.region.x).fills(),
+                    PlaceDescAxis::shifted(self.region.y).fills(),
+                )
+                .rel_base(Axis::X, self.frame.x.len()),
             );
             Size::LEFTOVER
         }
@@ -1136,11 +1134,10 @@ fn padding_and_stack_boxes_follow_the_region_without_drawing_again() {
         fn draw(&mut self, painter: &mut Painter) -> Size {
             painter.widget_at(
                 &self.child,
-                [
-                    Place::Fill(Part::From(self.region.x)),
-                    Place::Fill(Part::From(self.region.y)),
-                ],
-                None,
+                PlaceDesc::new(
+                    PlaceDescAxis::shifted(self.region.x).fills(),
+                    PlaceDescAxis::shifted(self.region.y).fills(),
+                ),
             );
             Size::LEFTOVER
         }
@@ -1224,14 +1221,14 @@ 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,
-                [
-                    Place::Fill(Part::From(UiSpan::new(
+                PlaceDesc::new(
+                    PlaceDescAxis::shifted(UiSpan::new(
                         Len::px(self.start),
                         Len::px(self.start + 200.0),
-                    ))),
-                    Place::Fill(Part::From(UiSpan::FULL)),
-                ],
-                None,
+                    ))
+                    .fills(),
+                    PlaceDescAxis::shifted(UiSpan::FULL).fills(),
+                ),
             );
             Size::LEFTOVER
         }
@@ -1266,11 +1263,10 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
             painter
                 .widget_at(
                     &self.child,
-                    [
-                        Place::Within(Part::From(self.region.x)),
-                        Place::Within(Part::From(self.region.y)),
-                    ],
-                    None,
+                    PlaceDesc::new(
+                        PlaceDescAxis::shifted(self.region.x),
+                        PlaceDescAxis::shifted(self.region.y),
+                    ),
                 )
                 .size()
         }
@@ -1286,11 +1282,10 @@ fn changing_regions_keep_fractional_reports_and_numeric_dependencies_valid() {
                 painter
                     .widget_at(
                         &self.child,
-                        [
-                            Place::Fill(Part::From(self.region.x)),
-                            Place::Fill(Part::From(self.region.y)),
-                        ],
-                        None,
+                        PlaceDesc::new(
+                            PlaceDescAxis::shifted(self.region.x).fills(),
+                            PlaceDescAxis::shifted(self.region.y).fills(),
+                        ),
                     )
                     .size(),
             );