multiboot2: streamline before release #993

Job	Run time
build (msrv) / rust	18s
build (nightly) / rust	22s
build (stable) / rust	15s
build no_std (stable) [Windows] / rust	1m 1s
build no_std (stable) [w/o builder] / rust	19s
style (stable) / rust	16s
build no_std (stable) / rust	12s
style (msrv) / rust	24s
build no_std (msrv) / rust	20s
tests with miri (nightly) / rust	1m 14s
build no_std (nightly) / rust	20s
style (nightly) / rust	26s
	5m 27s

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