block-padding: remove `PadType` and add `pad_detached` method #1910

Job	Run time
build (stable, thumbv7em-none-eabi)	21s
minimal-versions / minimal-versions	33s
build (1.85.0, wasm32-unknown-unknown)	20s
build (1.85.0, thumbv7em-none-eabi)	17s
build (stable, wasm32-unknown-unknown)	17s
test (1.85.0)	20s
test (stable)	21s
	2m 29s

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