Introduce `SparseMerkleTree` trait #858

Job	Run time
build stable no-std for wasm32-unknown-unknown	14s
clippy nightly on ubuntu	50s
rustfmt nightly on ubuntu	14s
build nightly no-std for wasm32-unknown-unknown	19s
test stable on ubuntu with --features default,serde	1m 9s
test stable on ubuntu with --no-default-features	24s
test nightly on ubuntu with --features default,serde	54s
test nightly on ubuntu with --no-default-features	34s
	4m 38s

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