ctutils: `subtle` interop #17

Job	Run time
test (stable)	20s
test (1.85.0)	24s
test-careful	55s
build (1.85.0, wasm32-unknown-unknown)	14s
build (stable, wasm32-unknown-unknown)	12s
build (stable, thumbv7em-none-eabi)	10s
build (1.85.0, thumbv7em-none-eabi)	21s
minimal-versions / minimal-versions	20s
	2m 56s

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