dsa: migrate to `crypto-bigint` #159

Job	Run time
build (thumbv7em-none-eabi, 1.72.0)	18s
build (thumbv7em-none-eabi, stable)	11s
build (wasm32-unknown-unknown, 1.72.0)	18s
test (1.72.0)	18s
build (wasm32-unknown-unknown, stable)	16s
test (stable)	23s
	1m 44s

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