Changes to bring indexes into better alignment with trees:
Rename index_empty(), index_path(), and index_size() to empty(), path(), and size().
Add max_size(), node_size(), max_cache_size(), and max_cache_megabytes() functions.
Add heterogeneous_key_test(), but commented out as it is not yet supported by implementation.
Work in process extending reference.html to cover indexes.
Add table specifying the header for each container class.