killed final stops in Sledgehammer and friends
prefer infix operations;
clarified modules; --HG-- rename : src/Pure/Concurrent/simple_thread.ML => src/Pure/Concurrent/standard_thread.ML rename : src/Pure/Concurrent/simple_thread.scala => src/Pure/Concurrent/standard_thread.scala
further reduced dependency on legacy async thread manager
more explicit thread identification;
more explicit indication of Async_Manager_Legacy as Proof General legacy; --HG-- rename : src/HOL/Tools/Sledgehammer/async_manager.ML => src/HOL/Tools/Sledgehammer/async_manager_legacy.ML