7953 huge debugger st pharo application memory leak

[StevenCostiou]

Fixes #7953
I believe this is due to the fact that:

• debug sessions and other debugger model objects holds a reference to the exception that provoked a debugger opening
• the exceptions references the interrupted process and context that prevent garbage collection

This may be a side effect of pharo-spec/NewTools#136 where the debugging processes are never released after closing a debugger (under investigation).

With this fix, now the reproduction example of the issue gives the following result:

Trying the same on instances of StDebugger returns nil.

[Ducasse]

Thanks Steven! Let us try.
I would really like to know how we could have tools to find such bug.
Please archive this bug somewhere.