Implement `SimpleSmt::set_subtree` #811

Job	Run time
rustfmt	7s
Clippy with --features default,std,serde	26s
Test stable on ubuntu with --features default,std,serde	42s
Build stable on ubuntu with --no-default-features --target wasm32-unknown-unknown	22s
Clippy with --no-default-features	26s
Test stable on ubuntu with --no-default-features	43s
Build nightly on ubuntu with --no-default-features --target wasm32-unknown-unknown	25s
Test nightly on ubuntu with --features default,std,serde	55s
Test nightly on ubuntu with --no-default-features	49s
	4m 55s

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