Say what a child gets of a container's box in that box's own lengths

Three things the measurements asked for, all about how much a box that came
from an answer costs.

**A part in the box's own coordinates.** Saying "less eleven pixels at the
end" in frame lengths from the box's start means reading how long the box is,
and a container whose box is its own answer then depends on its own answer:
`Pad` drew sixty-four times in one resize frame at seed 13, chasing its own
width. `Part::Of` says the same thing as a part of the box, which composes
without a length -- pixels are pixels wherever the box lands -- and what a
child under it holds for maps back through that part onto the container's
own box rather than onto the frame.

**One axis of the box at a time.** `extent_len` pinned both axes, so a span
dividing one of them held for one length of the other as well, and a resize
broke every span whose cross-axis answer moved.

**No lazy placement.** Leaving a child's answer to be placed at the end of
the parent's draw, rather than as the child answers, was meant to save a
recomposition. It costs one instead: the drawing is put in the part first
and in the answer's box after, and where it does not hold for both that is
two drawings rather than one. Seed 1 at depth 8 went from 391 widget draws
on a resize to 29 with it gone. The test that pinned three draws for a
numeric leaf in a span goes with it.

Seed 1 at depth 8, widget draws / distinct widgets / update, against #18's
head and against the commit this branch started from:

| phase   | e44dea3      | 34cafb6      | here          |
| ---     | ---          | ---          | ---           |
| cold    | 369/261/10.6 | 463/274/13.3 | 516/288/12.0  |
| repaint | 1            | 1            | 1             |
| many    | 157/95/0.33  | 263/108/0.59 | 187/119/0.52  |
| size    | 16/12/0.018  | 3/3          | 3/3/0.010     |
| scroll  | 2/0.002      | 1            | 1/0.004       |
| resize  | 13/13/0.019  | 22/15/0.032  | 24/76/0.090   |

Seed 13 at depth 8 is where the protocol still costs: `many` 1091 draws
against #18's 524, and `resize` 2215 against a frame #18 does not draw at
all. Both are the same shape -- an answer measured in one box and drawn in
another -- and the handoff says where that comes from.

Checked: fmt, clippy with -D warnings, 108 suite tests, 20 core tests, the
11 generated cases, and the shrinker at 400 trees of depth 5, which fails
seeds 2 (repaint) and 108 (reorder).

src/widget/position/span.rs

@@ -13,7 +13,7 @@ impl Widget for Span {
         // The row: this span's own box, as a length of the frame 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.extent_len().axis(axis);
+        let far = painter.extent_len(axis);
         let along = |from: Len, to: Len| match self.dir.sign {
             Sign::Pos => UiSpan::new(from, to),
             Sign::Neg => UiSpan::new(far - to, far - from),
@@ -21,7 +21,7 @@ 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(None);
+        let across = Place::Within(Part::All);
         // A length for every child before their final slots are chosen. The
         // frame 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
@@ -30,7 +30,7 @@ impl Widget for Span {
         let mut cursor = Len::rel_min();
         let mut lens = Vec::with_capacity(self.children.len());
         for child in &self.children {
-            let room = Place::Within(Some(along(cursor, far)));
+            let room = Place::Within(Part::From(along(cursor, far)));
             let len = painter
                 .widget_at(child, UiRegion::FULL, axis.pair(room, across))
                 .len(axis);
@@ -108,7 +108,7 @@ impl Widget for Span {
             // Along the row the span says where the child goes, and that slot
             // is the drawing's box outright rather than something to place an
             // answer inside again.
-            let slot = Place::Fill(Some(along(from, start)));
+            let slot = Place::Fill(Part::From(along(from, start)));
             let placed = painter.widget_at(child, UiRegion::FULL, axis.pair(slot, across));
             if shrinks {
                 let used = placed.len(!axis);