Introduce `SparseMerkleTree` trait #908

Job	Run time
build stable no-std for wasm32-unknown-unknown	22s
build nightly no-std for wasm32-unknown-unknown	22s
test stable on ubuntu with --features default,serde	47s
clippy nightly on ubuntu	48s
rustfmt nightly on ubuntu	14s
test stable on ubuntu with --no-default-features	44s
test nightly on ubuntu with --features default,serde	46s
test nightly on ubuntu with --no-default-features	46s
	4m 49s

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