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).