fix: comments (#1923)

follow up to #1919

Author: joseph-isaacs

vortex-layout/src/layouts/struct_/eval_expr.rs

@@ -20,7 +20,6 @@ impl ExprEvaluator for StructReader {
         let field_readers: Vec<_> = partitioned
             .partitions
             .iter()
-            // TODO(joe): remove field from self.child
             .map(|partition| self.child(&partition.name.clone()))
             .try_collect()?;
 

vortex-layout/src/layouts/struct_/reader.rs

@@ -69,7 +69,7 @@ impl StructReader {
             .get(name)
             .ok_or_else(|| vortex_err!("Field {} not found in struct layout", name))?;
 
-        // TODO: think about a hashmap for large |fields|.
+        // TODO: think about a Hashmap<FieldName, OnceLock<Arc<dyn LayoutReader>>> for large |fields|.
         self.field_readers[idx].get_or_try_init(|| {
             let child_layout = self
                 .layout