srp

Reapply "srp: add HomeKit-compatible padding for u and M1/M2" #492

Job	Run time
test (stable)	2m 1s
build (1.85, thumbv7em-none-eabi)	23s
build (stable, wasm32-unknown-unknown)	22s
build (1.85, wasm32-unknown-unknown)	18s
build (stable, thumbv7em-none-eabi)	18s
test (1.85)	2m 10s
	5m 32s

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