multiboot2: massive refactoring, removed UB, Miri passes all tests #958

Job	Run time
build (stable) / rust	11s
build (msrv) / rust	19s
build (nightly) / rust	17s
build no_std (stable) [Windows] / rust	2m 20s
build no_std (stable) [w/o builder] / rust	14s
build no_std (stable) / rust	16s
style (stable) / rust	14s
tests with miri (nightly) / rust	1m 20s
build no_std (nightly) / rust	19s
style (nightly) / rust	22s
style (msrv) / rust	25s
build no_std (msrv) / rust	24s
	6m 41s

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