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/scroll.rs

@@ -14,7 +14,7 @@ impl Widget for Scroll {
         let container_len = painter.px_len(self.axis);
         // Asked in the whole viewport, then put at the scrolled offset.
         let answer_len = painter
-            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::All); 2])
+            .widget_at(&self.inner, [None; 2], [Place::Fill(Part::WHOLE); 2])
             .len(self.axis);
         let fixed = painter.to_px(Len::from_parts(answer_len.rel, answer_len.px), self.axis);
         self.container_len = container_len;
@@ -56,15 +56,17 @@ impl Widget for Scroll {
                 let start = Len::from_parts(Rel::ZERO, anchor - self.amt);
                 Part::From(UiSpan::new(start, start.offset(self.content_len)))
             }
-            false => Part::All,
+            false => Part::WHOLE,
         };
-        // The viewport is the inner's frame, so a fraction it declares or
+        // The viewport is the inner's rel base, so a fraction it declares or
         // reports is a fraction of what is on screen rather than of the
         // content box its own answer decided. Where it goes is the content
         // box, scrolled: its drawing moved there, not made again there.
         painter.place_at(
             &self.inner,
-            self.axis.pair(Place::Fill(content), Place::Fill(Part::All)),
+            [None; 2],
+            self.axis
+                .pair(Place::Fill(content), Place::Fill(Part::WHOLE)),
         );
         // What it occupies is its box, on both axes: it clips its content to
         // that box, so it can neither take less of one nor honestly ask for