variables from a shared bdd_dict.  Register Next variables as
anonymous variables.
(translate_dict::translate_dict, translate_dict::~translate_dict,
translate_dict::register_proposition,
translate_dict::register_a_variable,
translate_dict::register_next_variable,
translate_dict::dump, translate_dict::var_to_formula,
ltl_to_tgba_fm): Adjust.
(translate_dict::dict): New attribute.
(translate_dict::a_map, translate_dict::a_formula_map,
translate_dict::var_map, translate_dict::var_formula_map): Delete.