Make extensible

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This was @krakjoe 's idea.

I think we have a head start on this as the tracing code is already decoupled with the event handling code. See:

phpspy/phpspy.c

Line 262 in e9927c8

context.event_handler = event_handler_fout; /* TODO set based on option */

phpspy/event_fout.c

Line 12 in e9927c8

int event_handler_fout(struct trace_context_s *context, int event_type) {

Maybe we write an event_lua.c that is end-user scriptable?