block-padding: merge `Padding` and `RawPadding` traits #1892

Job	Run time
build (stable, thumbv7em-none-eabi)	18s
build (1.85.0, thumbv7em-none-eabi)	18s
build (stable, wasm32-unknown-unknown)	19s
build (1.85.0, wasm32-unknown-unknown)	18s
minimal-versions / minimal-versions	31s
test (stable)	17s
test (1.85.0)	17s
	2m 18s

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