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.

core/src/ui/active.rs

@@ -13,10 +13,10 @@ pub struct ActiveData {
     pub placement: UiRegion,
     /// What a fraction declared or reported under this widget is a fraction
     /// of, as a length of the window.
-    pub frame: UiVec2,
-    /// A frame its parent decided for it on each axis -- a row's slot, or
-    /// padding's frame less its pixels -- as a length of the window. `None`
-    /// forwards the parent's frame. What it declared is kept separately in
+    pub rel_base: UiVec2,
+    /// A rel base its parent decided for it on each axis -- a row's slot, or
+    /// padding's rel base less its pixels -- as a length of the window. `None`
+    /// forwards the parent's rel base. What it declared is kept separately in
     /// `declared` and is a fraction of whichever of the two reached it.
     pub narrow: [Option<Len>; 2],
     /// Where its drawing was put, and where it was asked, each as a part of
@@ -41,7 +41,7 @@ pub struct ActiveData {
     /// What the widget reported, in window-unit lengths.
     pub size: Size,
     /// The window and region reads that this drawing holds for, and the
-    /// frame and region it pinned.
+    /// rel base and region it pinned.
     pub holds: LayoutHolds,
     pub drawn: bool,
     pub parent: Option<WidgetId>,
@@ -61,7 +61,7 @@ pub struct ActiveData {
     /// The movable region its primitives are positioned through: its own when
     /// opted in, otherwise the nearest ancestor's.
     pub move_idx: MoveIdx,
-    /// The declared lengths whoever drew this widget resolved into its frame.
+    /// The declared lengths whoever drew this widget resolved into its rel base.
     /// A change to one moves a box this widget cannot fix by drawing again,
     /// and comparing them is what says so.
     pub declared: [Option<LayoutLen>; 2],