Assign cache slot based on which dimensions are known

This gives huge performance wins on deep trees. On my machine, an
improvement from 17s to 3ms (note change of unit) on the benchmark
with 10,000 nodes at a depth of 14

Author: nicoburns

src/compute/mod.rs

@@ -112,6 +112,7 @@ fn compute_node_layout(
     };
 
     // Cache result
+    let cache_slot = (known_dimensions.width.is_some() as usize) + (known_dimensions.height.is_some() as usize * 2);
     *tree.cache_mut(node, cache_slot) =
         Some(Cache { known_dimensions, available_space, run_mode: cache_run_mode, cached_size: computed_size });