Add a persistent state table that uses a destructor upon reloads

[kstrafe]

There is a need for semi-persistence in some cases. Especially threads may make use
of this functionality: the thread needs to be reloaded but its previous instance must
be killed. For this we need to persist the variable until after the reload and then
kill the thread

An example:

(make-persistent-state-destructor 'name (lambda () (thread (lambda ()
                                          (let loop ()
                                            (sleep 1)
                                            (displayln (current-seconds))
                                            (loop)))))
                                  kill-thread)

On my webserver I have a thread that performs certain IO operations periodically, however,
I want it to be reloaded (and the old instance killed) whenever I change a file. This would
be a very good addition to the library.