refactor: Pass `this` pointer as first param to s11n callbacks. #4821

Job	Run time
docker-clusterfuzz	27s
docker-esp32	3m 20s
docker-bootstrap-node	1m 5s
docker-fuzzer	10s
docker-toxcore-js	2m 1s
docker-win32	8s
docker-win64	7s
docker-bootstrap-node-websocket	11s
	7m 29s

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