multiboot2: streamlined tag getters #1047

Job	Run time
build (nightly) / rust	26s
build (stable) / rust	25s
build (msrv) / rust	21s
style (msrv) / rust	24s
build no_std (msrv) / rust	23s
build no_std (stable) / rust	19s
build no_std (stable) [w/o builder] / rust	26s
build no_std (stable) [Windows] / rust	1m 53s
style (stable) / rust	15s
build no_std (nightly) / rust	35s
tests with miri (nightly) / rust	1m 41s
style (nightly) / rust	34s
	7m 42s

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