init multiboot2-common and use same safe memory abstractions also in multiboot2-header crate #972

Job	Run time
build (msrv) / rust	24s
build (stable) / rust	16s
build (nightly) / rust	24s
build no_std (stable) [Windows] / rust	34s
build no_std (stable) [w/o builder] / rust	10s
build no_std (stable) / rust	14s
style (stable) / rust	18s
build no_std (nightly)	0s
tests with miri (nightly)	0s
style (nightly)	0s
style (msrv) / rust	33s
build no_std (msrv) / rust	26s
	3m 19s

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