Implement `SimpleSmt::set_subtree` #810

Job	Run time
rustfmt	5s
Clippy with --features default,std,serde	26s
Test stable on ubuntu with --features default,std,serde	46s
Clippy with --no-default-features	20s
Build stable on ubuntu with --no-default-features --target wasm32-unknown-unknown	21s
Test stable on ubuntu with --no-default-features	43s
Build nightly on ubuntu with --no-default-features --target wasm32-unknown-unknown	27s
Test nightly on ubuntu with --features default,std,serde	48s
Test nightly on ubuntu with --no-default-features	51s
	4m 47s

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