Implement `Smt` struct (replacement to `TieredSmt`) #941

Job	Run time
Verify the docs on stable	36s
build stable no-std for wasm32-unknown-unknown	24s
clippy nightly on ubuntu	49s
rustfmt nightly on ubuntu	14s
build nightly no-std for wasm32-unknown-unknown	28s
test stable on ubuntu with --features default,serde	45s
test stable on ubuntu with --no-default-features	48s
test nightly on ubuntu with --features default,serde	47s
test nightly on ubuntu with --no-default-features	44s
	5m 35s

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