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/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)),
         );