block-buffer: improve safety comments, use `MaybeUninit` for internal… #1671

Job	Run time
build (1.57.0, thumbv7em-none-eabi)	12s
build (1.57.0, wasm32-unknown-unknown)	14s
test (1.57.0)	18s
build (stable, thumbv7em-none-eabi)	14s
test (stable)	8s
build (stable, wasm32-unknown-unknown)	20s
minimal-versions / minimal-versions	15s
	1m 41s

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