Improve "stored value" reporting

[madmaxoft]

If a plugin stores a parameter value from a callback for later, it should be possible to provide better diagnostics about where exactly the parameter was stored. If the callback is a 100+ line function, it can be quite difficult for a human to analyze it.

Using the debug library it should be possible to examine the function's locals and upvalues, seeing which one contains the saved instance. For example if the callback creates a new closure with the value captured into the closure, it should be possible to see that - there'll be a local value (the closure) of type function whose upvalue would be equal to the saved object.

It may be necessary to re-execute the handler in order to be able to see its internals via the debug library, possibly even installing a debug hook so that code can be executed while the function is still on the stack. Perhaps this could be made into a new detection mode that installs a hook for each callback's return and checks all its locals / upvalues.