init multiboot2-common and use same safe memory abstractions also in multiboot2-header crate #970

Job	Run time
build (msrv) / rust	20s
build (nightly) / rust	20s
build (stable) / rust	12s
build no_std (stable) [w/o builder] / rust	9s
build no_std (stable) / rust	19s
build no_std (stable) [Windows] / rust	36s
style (stable) / rust	22s
style (msrv) / rust	23s
build no_std (msrv) / rust	25s
style (nightly)	0s
tests with miri (nightly)	0s
build no_std (nightly)	0s
	3m 6s

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