Return the size from draw, and fold placing back into drawing

Review response.

`Painter::set_size` is gone: `Widget::draw` returns the `Size` instead, so
a widget that does not say what it used cannot compile rather than
panicking at the widget that forgot. That also settles setting it twice --
a branch that learns something late just returns a different value.

`Painter::place` and `UiRenderState::place` are gone too. `draw_inner`
already tried to reuse an active widget's drawing before redrawing it, so
`place` was `widget_within` with its own bookkeeping bolted on; drawing a
child a second time now *is* how a parent puts it where it belongs, and a
child is deduplicated in `children` because listing one twice would move
it twice. The unification also drops `place`'s use of `ActiveData::layer`,
which is the layer a widget's own `child_layer()` left the painter on
rather than the layer it was drawn into.

What `place` did unconditionally and `widget_within` did not is record the
size dependency, so `Painter::size_hint` now records one: reading a
child's length to lay out around it is reading its size, whether it came
from a draw or from a hint. `tests/retained.rs` has a parent that only
ever reads the hint, which is the case no existing widget exercises.

`()` sizes itself `Size::default()` -- rest -- rather than zero, so it is
a gap that takes an even share of a span; `WidgetPtr` with nothing in it
does the same, since it is the same situation. `Widget::on_resize`'s
default body said `Translate` while the enum's `#[default]` said `Redraw`;
it now defers to the enum.

`was` is `old` throughout, the `OnResize` variant comments are gone, and
so are two empty `impl` blocks.

Checked: fmt, clippy and 27 tests across the workspace; `tabs` on each of
its five tabs, and `tabs` with a replay that adds two images, all
byte-identical to `upstream/main`; `view` and `minimal` likewise; and the
`text` example rendered at 1920x1200 and 900x1200 to see the paragraph
reflow and its container follow.

src/widget/position/span.rs

@@ -8,7 +8,7 @@ pub struct Span {
 }
 
 impl Widget for Span {
-    fn draw(&mut self, painter: &mut Painter) {
+    fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
         // A length for every child before any is placed: from its own hint
         // where it has one, and from drawing it where it does not.
@@ -42,7 +42,7 @@ impl Widget for Span {
             if self.dir.sign == Sign::Neg {
                 region.flip(axis);
             }
-            let used = painter.place(child, region).axis(!axis);
+            let used = painter.widget_within(child, region).size().axis(!axis);
             // TODO: rel shouldn't do this, but no easy way before actually calculating pixels
             if used.rel > 0.0 || used.rest > 0.0 {
                 ortho = Len::REST;
@@ -56,7 +56,7 @@ impl Widget for Span {
             true => total,
             false => Len::default(),
         };
-        painter.set_size(Size::from_axis(axis, along, ortho));
+        Size::from_axis(axis, along, ortho)
     }
 }