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/offset.rs

@@ -7,14 +7,8 @@ pub struct Offset {
 
 impl Widget for Offset {
     fn draw(&mut self, painter: &mut Painter) -> Size {
-        // The whole of this widget's box, moved: the frame passes through, so
-        // what the child declares or reports means the same as it would
-        // without the offset.
-        let moved = |len: Len, amt: Len| Place::Within(Part::From(UiSpan::new(amt, len + amt)));
-        let place = [
-            moved(painter.region_len(Axis::X), self.amt.x),
-            moved(painter.region_len(Axis::Y), self.amt.y),
-        ];
-        painter.widget_at(&self.inner, [None; 2], place).size()
+        painter
+            .widget_within(&self.inner, UiRegion::FULL.offset(self.amt))
+            .size()
     }
 }