Say narrow_rel_base, and let a container pass None for it

`narrow` said what the argument did to a value it never named, so a reader
had to go and find out which value. It is `narrow_rel_base`, and the
resolved one stays the bare `rel_base` -- which is also the only one in
`Painter`, `Placing` and `LayoutHolds`, where there is nothing to tell it
apart from.

It takes `impl Into<Option<[Option<Len>; 2]>>`, so a container that does not
narrow anything writes `None` rather than `[None; 2]`, and `Span` builds the
one case that does with a `then` instead of a mutable array.

Cold layout is byte-identical to `84dad21`.

src/random.rs

@@ -120,7 +120,7 @@ impl Widget for Branch {
         let cut = Len::from_parts(Rel::ZERO, Px::from_int(40));
         let top = Place::Within(Part::From(UiSpan::new(Len::ZERO, cut)));
         let measured = painter
-            .widget_at(&self.probe, [None; 2], [Place::Within(Part::WHOLE), top])
+            .widget_at(&self.probe, None, [Place::Within(Part::WHOLE), top])
             .len(Axis::X);
         let len = measured.apply_leftover();
         let px = painter.to_px(len, Axis::X);
@@ -137,8 +137,8 @@ impl Widget for Branch {
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > threshold {
-            true => painter.widget_at(&self.wide, [None; 2], place),
-            false => painter.widget_at(&self.narrow, [None; 2], place),
+            true => painter.widget_at(&self.wide, None, place),
+            false => painter.widget_at(&self.narrow, None, place),
         };
         Size::LEFTOVER
     }

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::WHOLE); 2])
+            .widget_at(&self.inner, None, [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;
@@ -64,7 +64,7 @@ impl Widget for Scroll {
         // box, scrolled: its drawing moved there, not made again there.
         painter.place_at(
             &self.inner,
-            [None; 2],
+            None,
             self.axis
                 .pair(Place::Fill(content), Place::Fill(Part::WHOLE)),
         );

src/widget/position/span.rs

@@ -36,7 +36,7 @@ impl Widget for Span {
                 None => {
                     let room = Place::Within(Part::From(along(cursor, far)));
                     painter
-                        .widget_at(child, [None; 2], axis.pair(room, across))
+                        .widget_at(child, None, axis.pair(room, across))
                         .len(axis)
                 }
             };
@@ -121,10 +121,8 @@ impl Widget for Span {
             // room is put there as it is, and one not made yet is made here.
             let slot = along(from, start);
             let place = axis.pair(Place::Fill(Part::From(slot)), across);
-            let mut narrow = [None; 2];
-            if len.leftover > Weight::ZERO && shares {
-                narrow[axis as usize] = Some(slot.len());
-            }
+            let narrow =
+                (len.leftover > Weight::ZERO && shares).then(|| axis.pair(Some(slot.len()), None));
             let used = painter.place_at(child, narrow, place).len(!axis);
             if shrinks {
                 // Choosing between a fixed and a relative length from the

src/widget/position/stack.rs

@@ -23,7 +23,7 @@ impl Widget for Stack {
             Some((i, child)) => {
                 painter.child_layer_at(i);
                 painter
-                    .widget_at(child, [None; 2], [Place::Fill(Part::WHOLE); 2])
+                    .widget_at(child, None, [Place::Fill(Part::WHOLE); 2])
                     .size()
             }
             None => Size::LEFTOVER,
@@ -45,7 +45,7 @@ impl Widget for Stack {
                 continue;
             }
             painter.child_layer_at(i);
-            painter.widget_at(child, [None; 2], place);
+            painter.widget_at(child, None, place);
         }
         size
     }