Carry a length as a rule beside a widget, not a widget around it

`.width()` built a `SetSize` whose whole job was to answer `size_hint`, so
every declared length cost a widget, an `ActiveData` and a link of chain to
say one number. It is now a `SizeRule` per axis on `WidgetData`, beside
`region_node`, resolved by `Painter` where the widget is drawn. `SetSize`
and `MaxSize` are gone; `MaxSize` had no caller but its own builders.

That settles which of two answers is the size. A rule wins on the axis it
names and the `Size` returned by `draw` answers the rest, applied once in
`draw_inner` rather than by each widget that could carry one -- so the
widget under a rule never learns of it. `Painter::size_hint` reads the rule
first for the same reason: a rule that beats what a widget would draw has
to beat what it says about itself.

`declared_lens` still falls back to a non-leftover `size_hint`, which is
how an image or a gap gets its own pixel size rather than the whole offer.
That is the offer's business rather than a declaration's, and it falls away
when a widget occupies its reported size inside the box it was offered.

`known` and `declared` are separate because a share is a length to whoever
divides one and not to whoever composes a box: `.width(leftover(3))` is
known without drawing but cannot narrow anything.

Checked: fmt, clippy, 85 tests, and 100 generated seeds agreeing warm
against cold in 67.6 s. `minimal`, `text` and `view` render byte-identical
at 1920x1200; `tabs` differs only in the widget count it prints about
itself, which is two wrapper types smaller.

tests/unsettled.rs

@@ -15,12 +15,7 @@ use iris::prelude::*;
 fn plant(h: &mut Harness) -> Vec<WidgetId> {
     let plain = wtext("Wrapping").size(16).wrap(false).add(&mut h.rsc);
     let wrapped = wtext("Wrapping shapes").size(16).wrap(true).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: wrapped.add_strong(&mut h.rsc),
-        x: Some(Len::px(76.0)),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let sized = wrapped.width(76).add(&mut h.rsc);
     let aligned = Aligned {
         inner: sized.add_strong(&mut h.rsc),
         align: Align {
@@ -110,12 +105,7 @@ fn plant_fixed(h: &mut Harness) -> Vec<WidgetId> {
     }
     .add(&mut h.rsc);
     let inner = (aligned,).span(Dir::RIGHT).add(&mut h.rsc);
-    let sized = SetSize {
-        inner: inner.add_strong(&mut h.rsc),
-        x: Some(Len::px(189.0)),
-        y: Some(Len::px(176.0)),
-    }
-    .add(&mut h.rsc);
+    let sized = inner.sized((189, 176)).add(&mut h.rsc);
     let filler = rect(Color::RED).add(&mut h.rsc);
     let root = (filler, sized).span(Dir::RIGHT).add(&mut h.rsc);
     h.state.root = Some(root.add_strong(&mut h.rsc));
@@ -235,12 +225,7 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
     }
     .add(&mut h.rsc);
     let block = rect(Color::RED).add(&mut h.rsc);
-    let fixed = SetSize {
-        inner: block.add_strong(&mut h.rsc),
-        x: Some(Len::px(87.0)),
-        y: None,
-    }
-    .add(&mut h.rsc);
+    let fixed = block.width(87).add(&mut h.rsc);
     let mut outer_children: Vec<StrongWidget> =
         vec![fixed.add_strong(&mut h.rsc), inner.add_strong(&mut h.rsc)];
     if swapped {
@@ -253,12 +238,9 @@ fn plant_scrolled(h: &mut Harness, swapped: bool) -> (Vec<WidgetId>, [WeakWidget
         ortho: OrthoSize::Children,
     }
     .add(&mut h.rsc);
-    let through = SetSize {
-        inner: outer.add_strong(&mut h.rsc),
-        x: None,
-        y: None,
-    }
-    .add(&mut h.rsc);
+    // Carried no rule even before rules were a property: it is here to be a
+    // widget between the span and the scroll, not to declare anything.
+    let through = (outer,).span(Dir::RIGHT).add(&mut h.rsc);
     let scroll = Scroll::new(through.add_strong(&mut h.rsc), Axis::X).add(&mut h.rsc);
     h.state.root = Some(scroll.add_strong(&mut h.rsc));
     (