Make `SmtProof` cloneable #995

Job	Run time
Verify the docs on stable	53s
build stable no-std for wasm32-unknown-unknown	21s
clippy nightly on ubuntu	46s
rustfmt nightly on ubuntu	13s
build nightly no-std for wasm32-unknown-unknown	23s
test stable on ubuntu with --features default,serde	1m 1s
test stable on ubuntu with --no-default-features	42s
test nightly on ubuntu with --features default,serde	49s
test nightly on ubuntu with --no-default-features	45s
	5m 53s

Provide feedback