init multiboot2-common and use same safe memory abstractions also in multiboot2-header crate #974

Job	Run time
build (nightly) / rust	23s
build (msrv) / rust	44s
build (stable) / rust	31s
build no_std (nightly)	0s
style (nightly)	0s
tests with miri (nightly)	0s
build no_std (stable) / rust	12s
build no_std (stable) [w/o builder] / rust	13s
build no_std (stable) [Windows] / rust	2m 8s
style (stable) / rust	14s
build no_std (msrv) / rust	25s
style (msrv) / rust	22s
	5m 12s

Provide feedback