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`.

tests/cases/determinism.rs

@@ -24,15 +24,15 @@ impl Widget for BranchesOnMeasurement {
         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 px = painter.to_px(measured.apply_leftover(), Axis::X);
 
         let below = Place::Within(Part::From(UiSpan::new(cut, painter.region_len(Axis::Y))));
         let place = [Place::Within(Part::WHOLE), below];
         match px > Px::from_f32(self.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
     }