Remove usage of unstable box syntax, except in the script crate

… because there’s a lot of it,
and script still uses any other unstable features anyway.

components/layout/block.rs

@@ -537,7 +537,7 @@ impl BlockFlow {
                 None => ForceNonfloatedFlag::ForceNonfloated,
             }),
             fragment: fragment,
-            float: float_kind.map(|kind| box FloatedBlockInfo::new(kind)),
+            float: float_kind.map(|kind| Box::new(FloatedBlockInfo::new(kind))),
             flags: BlockFlowFlags::empty(),
         }
     }