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/span.rs

@@ -10,7 +10,7 @@ pub struct Span {
 impl Widget for Span {
     fn draw(&mut self, painter: &mut Painter) -> Size {
         let axis = self.dir.axis;
-        // The row: this span's own box, as a length of the frame its children
+        // The row: this span's own box, as a length of the rel base its children
         // are laid out against. Its start is nothing's business -- a slot is
         // a length from it -- so what this reads is the length alone.
         let far = painter.region_len(axis);
@@ -21,29 +21,23 @@ impl Widget for Span {
         // Across itself the child sits where its own alignment says, in the
         // whole of the row: a span is what contains its children there, and
         // nothing divides that axis.
-        let across = Place::Within(Part::All);
+        let across = Place::Within(Part::WHOLE);
         // A length for every child before their final slots are chosen: from
-        // a hint where one says, and from drawing otherwise. The frame passes
+        // a hint where one says, and from drawing otherwise. The rel base passes
         // through unchanged, so `rel(0.5)` is half the area this span was
         // given whatever else is in it and wherever this child sits among
         // them; what a drawn child is asked in is the room left from the
         // cursor, because a text has to wrap at the width actually there.
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
-        let mut drawn_across = Vec::with_capacity(self.children.len());
         for child in &self.children {
             let len = match painter.size_hint(child, axis) {
-                Some(len) => {
-                    drawn_across.push(None);
-                    len
-                }
+                Some(len) => len,
                 None => {
                     let room = Place::Within(Part::From(along(cursor, far)));
-                    let size = painter
+                    painter
                         .widget_at(child, [None; 2], axis.pair(room, across))
-                        .size();
-                    drawn_across.push(Some(size.axis(!axis)));
-                    size.axis(axis)
+                        .len(axis)
                 }
             };
             cursor.px += len.px + self.gap;
@@ -63,7 +57,7 @@ impl Widget for Span {
         );
 
         // What is left for the shares to divide: the row less everything
-        // fixed, as a length of the frame rather than a number of pixels.
+        // fixed, as a length of the rel base rather than a number of pixels.
         let room = far - Len::from_parts(total.rel, total.px);
         // Whether anything is left over is a question in pixels: `rel(0.5)`
         // beside 300 px is full at 600 and overfull at 400. Asked of `room`
@@ -100,7 +94,7 @@ impl Widget for Span {
         let mut taken = Weight::ZERO;
         let mut start = Len::rel_min();
         let mut ortho = LayoutLen::ZERO;
-        for ((child, &len), &across_len) in self.children.iter().zip(&lens).zip(&drawn_across) {
+        for (child, &len) in self.children.iter().zip(&lens) {
             // A child asking for nothing but a part of what is left over,
             // when nothing is, is not drawn at all. One that also asked for
             // pixels or a fraction keeps those and overflows.
@@ -121,7 +115,7 @@ impl Widget for Span {
             // Along the row the span says where the child goes, and that slot
             // is the child's box outright rather than something to place an
             // answer inside again. A share is decided here and nowhere
-            // else: its slot narrows its frame, and the child is asked in
+            // else: its slot narrows its rel base, and the child is asked in
             // it, since a text wraps at the width it is actually given. A
             // fixed child's slot is its own answer, so a drawing made in the
             // room is put there as it is, and one not made yet is made here.
@@ -131,13 +125,7 @@ impl Widget for Span {
             if len.leftover > Weight::ZERO && shares {
                 narrow[axis as usize] = Some(slot.len());
             }
-            let used = match (across_len, narrow[axis as usize]) {
-                (Some(across_len), None) => {
-                    painter.place_at(child, place);
-                    across_len
-                }
-                _ => painter.widget_at(child, narrow, place).len(!axis),
-            };
+            let used = painter.place_at(child, narrow, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the
                 // span's own eventual width admits multiple fixed points.