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/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