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`.

src/widget/position/span.rs

@@ -21,7 +21,7 @@ impl Widget for Span {
         // Across itself the child sits where its own alignment says, in the
         // whole of the row: a span is what contains its children there, and
         // nothing divides that axis.
-        let across = Place::Within(Part::WHOLE);
+        let across = PlaceDescAxis::WHOLE;
         // A length for every child before their final slots are chosen: from
         // a hint where one says, and from drawing otherwise. The rel base passes
         // through unchanged, so `rel(0.5)` is half the area this span was
@@ -34,9 +34,9 @@ impl Widget for Span {
             let len = match painter.size_hint(child, axis) {
                 Some(len) => len,
                 None => {
-                    let room = Place::Within(Part::From(along(cursor, far)));
+                    let room = PlaceDescAxis::shifted(along(cursor, far));
                     painter
-                        .widget_at(child, axis.pair(room, across), None)
+                        .widget_at(child, PlaceDesc::from_axis(axis, room, across))
                         .len(axis)
                 }
             };
@@ -120,10 +120,12 @@ impl Widget for Span {
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
             let slot = along(from, start);
-            let place = axis.pair(Place::Fill(Part::From(slot)), across);
-            let narrow =
-                (len.leftover > Weight::ZERO && shares).then(|| axis.pair(Some(slot.len()), None));
-            let used = painter.place_at(child, place, narrow).len(!axis);
+            let slot_place = PlaceDescAxis::shifted(slot).fills();
+            let mut place = PlaceDesc::from_axis(axis, slot_place, across);
+            if len.leftover > Weight::ZERO && shares {
+                place = place.rel_base(axis, slot.len());
+            }
+            let used = painter.place_at(child, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.