block-buffer: improve `split_blocks` implementations (#1220) #1886

Job	Run time
build (stable, thumbv7em-none-eabi)	16s
minimal-versions / minimal-versions	33s
test (stable)	18s
test (1.85.0)	16s
build (1.85.0, wasm32-unknown-unknown)	17s
build (stable, wasm32-unknown-unknown)	21s
build (1.85.0, thumbv7em-none-eabi)	20s
	2m 21s

Provide feedback