Allow padding up to MAX_BLOCK_SIZE #728

Job	Run time
build (i686-unknown-linux-gnu, ubuntu-latest, stable, sudo apt update && sudo apt install gcc-mul...	43s
build (x86_64-apple-darwin, macos-latest, stable)	46s
build (x86_64-pc-windows-msvc, windows-latest, stable)	1m 33s
no_std (1.81.0, thumbv7em-none-eabi)	1m 7s
no_std (1.81.0, wasm32-unknown-unknown)	1m 2s
no_std (stable, thumbv7em-none-eabi)	1m 4s
test (1.81.0)	2m 41s
no_std (stable, wasm32-unknown-unknown)	1m 3s
test (stable)	2m 42s
	12m 41s

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