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/trace_unsettled.rs

@@ -10,12 +10,7 @@ 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 {
@@ -108,12 +103,7 @@ 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));