Bump `crypto-bigint` to v0.6 #12

Job	Run time
clippy	2m 22s
build-wasm (1.83.0, wasm32-unknown-unknown)	21s
rustfmt	12s
build-wasm (stable, wasm32-unknown-unknown)	22s
cargo-careful	1m 33s
slow-tests (i686-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	2m 44s
slow-tests (x86_64-unknown-linux-gnu, stable, sudo apt update && sudo apt install gcc-multilib)	3m 6s
test-and-coverage	56s
	11m 36s

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