Stringify Thread.MutexKey in backtraces
Imported from https://youtrack.roxen.com/issue/PIKE-202
Reported by Jonas Walldén jonasw@roxen.com
The improved _sprintf() in Thread.MutexKey is not visible in backtraces since that code excludes certain objects to avoid unwanted references. It would be helpful if the stringified form of the object is substituted instead in low_backtrace() (where it currently inserts a FakeObject placeholder).