Add missing methods to `Smt` #970

Job	Run time
Verify the docs on stable	42s
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	22s
test stable on ubuntu with --features default,serde	42s
test stable on ubuntu with --no-default-features	39s
test nightly on ubuntu with --features default,serde	48s
test nightly on ubuntu with --no-default-features	45s
	5m 18s

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