Format a list of utils::person objects into a compact string, merging roles
per person and normalizing names. Contributors without explicit roles are
assigned "ctb". If NULL or ":unas" is supplied, returns ":unas".
fix_contributor(contributors = NULL)A single character string, e.g. "{Jane Doe [dtm, ctb]} and {John Smith [ctb]}".
A vector or list of person objects, or NULL, or the
character string ":unas".