Changes

Summary

1. merged
2. clarified signature: retain original Poly/ML names Thread.Thread, Thread.Mutex, Thread.ConditionVar and de-emphasize them for Isabelle/ML;
3. add ML_system_pp for type Isabelle_Thread.T;
4. more explicit type Isabelle_Thread.T; total operation Isabelle_Thread.self: upgrade raw ML threads implicitly;
5. discontinue somewhat pointless thread tracing/debugging: without PIDE command context, messages are not shown, and Exn.trace hardly works anyway (see also de20fccf6509 and 447972249785); prefer Isabelle_Thread.fork;
6. tuned whitespace;