multiboot2: streamline before release #991

Job	Run time
build (nightly) / rust	19s
build (stable) / rust	16s
build (msrv) / rust	20s
build no_std (stable) / rust	13s
style (stable) / rust	15s
build no_std (stable) [w/o builder] / rust	17s
build no_std (stable) [Windows] / rust	40s
build no_std (msrv) / rust	25s
style (msrv) / rust	23s
build no_std (nightly) / rust	19s
tests with miri (nightly) / rust	1m 6s
style (nightly) / rust	21s
	4m 54s

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