dsa: migrate to `crypto-bigint` #876

Job	Run time
build (thumbv7em-none-eabi, 1.81.0)	14s
build (thumbv7em-none-eabi, stable)	19s
test (1.81.0)	16s
build (wasm32-unknown-unknown, 1.81.0)	14s
test (stable)	12s
build (wasm32-unknown-unknown, stable)	8s
	1m 23s

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