moved type bstring from name_space.ML to binding.ML -- it is the primitive concept behind bindings;
moved separator/is_qualified from binding.ML back to name_space.ML -- only name space introduces an explicit notation for qualified names;
type binding: maintain explicit qualifier, indepently of base name;
tuned signature of Binding: renamed name_pos to make, renamed base_name to name_of, renamed map_base to map_name, added mandatory flag to qualify, simplified map_prefix (formerly unused);
Binding.str_of: include markup with position properties;
misc tuning;