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/offset.rs

@@ -8,7 +8,7 @@ pub struct Offset {
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         painter
-            .widget_within(&self.inner, UiRegion::FULL.offset(self.amt))
+            .widget_at(&self.inner, UiRegion::FULL.offset(self.amt))
             .size()
     }
 }