(Vconcord_ds_hash_table): New variable.

(Vconcord_ds_hash_table): New variable.
(Vconcord_genre_hash_table): New variable.
(Fconcord_open_ds): Use Vconcord_ds_hash_table.
(Fconcord_ds_directory): New function.
(Fconcord_assign_genre): New function.
(Fconcord_genre_directory): New function.
(Fconcord_genre_ds): New function.
(Fconcord_feature_list): Allow to omit `ds'.
(Fconcord_make_object): Likewise.
(Fconcord_decode_object): Likewise.
(Fconcord_foreach_object_in_feature): Likewise.
(syms_of_concord): Add new builtin function Fconcord_ds_directory,
Fconcord_assign_genre, Fconcord_genre_directory and Fconcord_genre_ds.
(vars_of_concord): Setup Vconcord_ds_hash_table and
Vconcord_genre_hash_table.