Drop three things nothing reads, and say what a span makes scalable

`Axis::pair` and `RegionAlign::NEAR` arrived on this branch with no caller
and never got one. `Holds::contains` took `&self` where its five siblings on
the same `Copy` pair of pixels take `self`.

The comment beside a span's cross-axis accumulator said a scalable child
"makes Children scalable too"; `Children` names nothing here, and what it
makes scalable is the span.

core/src/ui/holds.rs

@@ -29,7 +29,7 @@ impl Holds {
         Self { lo: len, hi: len }
     }
 
-    pub const fn contains(&self, len: Px) -> bool {
+    pub const fn contains(self, len: Px) -> bool {
         len.raw() >= self.lo.raw() && len.raw() <= self.hi.raw()
     }