`Smt`: remove inner nodes when removing value #974

Job	Run time
clippy nightly on ubuntu	50s
rustfmt nightly on ubuntu	12s
build stable no-std for wasm32-unknown-unknown	18s
build nightly no-std for wasm32-unknown-unknown	22s
Verify the docs on stable	50s
test stable on ubuntu with --features default,serde	59s
test stable on ubuntu with --no-default-features	41s
test nightly on ubuntu with --features default,serde	54s
test nightly on ubuntu with --no-default-features	46s
	5m 52s

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