refactor: Add `mem` module to allow tests to override allocators. #3505

Job	Run time
docker-fuzzer	9s
docker-bootstrap-node	24s
docker-toxcore-js	2m 29s
docker-clusterfuzz	1m 38s
docker-esp32	43s
docker-win32	13s
docker-win64	9s
docker-bootstrap-node-websocket	0s
	5m 45s

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