Say rel base, and give containers back a box to hand over

`frame` named a length, not a rectangle, which was the one word in the
layout vocabulary that lied about its own shape. It is `rel_base`: what a
fraction a widget declares or reports is a fraction of.

Three API changes with it, all for containers that do one simple thing:

- `widget_within(id, region)` returns, taking a box in the widget's own
  coordinates and deriving the child's rel base from it. `Offset` and `Pad`
  are one call each again. `Offset` also stops reading `region_len`, which
  pinned its drawing to a box length it does not care about.
- `place_at` takes the rel base, returns the answer, and asks the child
  where there is no answer to re-express. Which of the two happens is the
  painter's to work out, so `Span`'s second pass is one call and its
  `drawn_across` bookkeeping is gone.
- `Part::All` is a `Part::WHOLE` constant rather than a variant, since it
  was exactly `Of(UiSpan::FULL)` and bought a separate arm in two matches.
  Measured at 0.07% of instructions retired against 0.04% run-to-run noise.

Cold layout is byte-identical to `84dad21` over 400 depth-5 trees.

src/widget/position/stack.rs

@@ -23,7 +23,7 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::All); 2])
+                    .widget_at(child, [None; 2], [Place::Fill(Part::WHOLE); 2])
                     .size()
             }
             None => Size::LEFTOVER,
@@ -37,7 +37,7 @@ impl Widget for Stack {
             let len = size.axis(axis);
             match len.leftover == Weight::ZERO {
                 true => Place::Fill(Part::Sized(Len::from_parts(len.rel, len.px))),
-                false => Place::Within(Part::All),
+                false => Place::Within(Part::WHOLE),
             }
         });
         for (i, child) in self.children.iter().enumerate() {