multiboot2: ElfSectionIter: implement ExactSizeIterator #1082

Job	Run time
build (nightly) / rust	49s
build (msrv) / rust	45s
build (stable) / rust	43s
build no_std (stable) [Windows] / rust	2m 32s
build no_std (stable) [w/o builder] / rust	24s
build no_std (stable) / rust	24s
build no_std (msrv) / rust	24s
style (msrv) / rust	20s
style (stable) / rust	20s
tests with miri (nightly) / rust	2m 40s
build no_std (nightly) / rust	19s
style (nightly) / rust	24s
	10m 4s

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