Add an optional 'lock' kwarg to callback that locks GC and scheduler. This
allows the callback to be invoked asynchronously in 'interrupt context',
for example as a signal handler.
Also add the 'cfun' member function to callback, that allows retrieving the
C callback function address. This is needed when the callback should be
set to a struct field.
See related #7373.
Signed-off-by: Amir Gonnen <amirgonnen@gmail.com>
cb332ddae8