Reduce PHP overhead in field_attach_load

field_attach_load() does a lot of "call a function, get an array result, and then iterate over the array to merge the results with others." e.g.:

    // Invoke the storage engine's hook_field_storage_load(): the field storage
    // engine loads the rest.
    $additions = module_invoke(variable_get('field_storage_module', 'field_sql_storage'), 'field_storage_load', $obj_type, $queried_objects, $age, $skip_fields);

    // First, merge the additions from the storage engine.
    foreach ($additions as $id => $obj_additions) {
      foreach ($obj_additions as $key => $value) {
        $queried_objects[$id]->$key = $value;
      }
    }

Instead, I suggest that we pass the collection array directly to each hook by reference and let them build it up directly, reducing data copying and iterations. Note that hook_field_attach_pre_load() already works this way.

Yes, this requires more cooperation among modules, but we already have hooks that let anyone stomp all over anything anyway, so...