block-buffer: improve safety comments, use `MaybeUninit` for internal… #1654

Job	Run time
build (1.41.0, thumbv7em-none-eabi)	19s
test (1.41.0)	11s
build (1.41.0, wasm32-unknown-unknown)	22s
test (stable)	7s
build (stable, thumbv7em-none-eabi)	16s
build (stable, wasm32-unknown-unknown)	5s
minimal-versions / minimal-versions	16s
	1m 36s

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