multiboot2: massive refactoring, removed UB, Miri passes all tests #950

Job	Run time
build (msrv) / rust	53s
build (stable) / rust	18s
build (nightly) / rust	18s
build no_std (stable) [Windows] / rust	2m 7s
build no_std (stable) / rust	14s
style (stable) / rust	27s
build no_std (stable) [w/o builder] / rust	44s
build no_std (nightly) / rust	25s
tests with miri (nightly) / rust	1m 2s
style (nightly) / rust	17s
style (msrv) / rust	19s
build no_std (msrv) / rust	20s
	7m 24s

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