Implement `Smt` struct (replacement to `TieredSmt`) #938

Job	Run time
clippy nightly on ubuntu	32s
rustfmt nightly on ubuntu	15s
build stable no-std for wasm32-unknown-unknown	19s
Verify the docs on stable	37s
build nightly no-std for wasm32-unknown-unknown	24s
test stable on ubuntu with --features default,serde	55s
test stable on ubuntu with --no-default-features	46s
test nightly on ubuntu with --features default,serde	49s
test nightly on ubuntu with --no-default-features	46s
	5m 23s

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