This allows the rust code to free up C++ resources allocated for a callback even
when the callback isn't executed (as opposed to requiring the callback to run
and at the end of the callback cleaning up all allocated resources).
Also add type-erased destructor registration to callback_t. This allows for
freeing variables allocated by the callback for debounce_t's
perform_with_callback() that don't end up having their completion called due to
a timeout.
