This adds a new WebAPIResource.serialize_object_list() method and

serialize_object_list_func argument to WebAPIResponsePaginated.

These allow an implementation to pre-process the list of items going

into a response, or to post-process or replace the serialized result.
The main use case is to allow resources to perform additional logic

across the list of items before or after serializing, such as

bulk-fetching extra data from cache or extensions hooks to include in

results.

Unit tests pass.