join_results: allow CRITICAL join of finished futures; added singleton join_result;
join_results: allow CRITICAL join of finished futures;
added singleton join_result;

MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle,MerelyAPseudonym/isabelle