Make signing and key generation C independent #1074

Job	Run time
clippy nightly on ubuntu	41s
clippy nightly on ubuntu	41s
Verify the docs on stable	38s
Verify the docs on stable	38s
build stable no-std for wasm32-unknown-unknown	22s
build stable no-std for wasm32-unknown-unknown	22s
rustfmt nightly on ubuntu	14s
rustfmt nightly on ubuntu	14s
build nightly no-std for wasm32-unknown-unknown	28s
build nightly no-std for wasm32-unknown-unknown	28s
test stable on ubuntu with --features default,serde	30m 2s
test stable on ubuntu with --features default,serde	43s
test stable on ubuntu with --no-default-features	46s
test stable on ubuntu with --no-default-features	46s
test nightly on ubuntu with --features default,serde	53s
test nightly on ubuntu with --features default,serde	53s
test nightly on ubuntu with --no-default-features	30m 1s
test nightly on ubuntu with --no-default-features	54s
	1h 9m 44s

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