message: Use constants for map fields
  Summary: Minor code cleanup.
diff: @@ -34,8 +34,8 @@ pub(crate) fn visit_generic_map<'a, K: Allocative + 'a, V: Allocative + 'a>(
  ) {
  let mut visitor = visitor.enter_unique(Key::new("data"), mem::size_of::<*const ()>());
  for (k, v) in entries {
  - visitor.visit_field(Key::new("key"), k);
  - visitor.visit_field(Key::new("value"), v);
  + visitor.visit_field(KEY_NAME, k);
  + visitor.visit_field(VALUE_NAME, v);
  }
  visitor.exit();
  }
  