block-buffer: improve `split_blocks` implementations (#1220) #1897

Job	Run time
build (stable, wasm32-unknown-unknown)	21s
test (stable)	24s
build (stable, thumbv7em-none-eabi)	22s
test (1.85.0)	16s
minimal-versions / minimal-versions	29s
build (1.85.0, thumbv7em-none-eabi)	14s
build (1.85.0, wasm32-unknown-unknown)	16s
	2m 22s

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