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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>