Introduce `SparseMerkleTree` trait #886

Job	Run time
build stable no-std for wasm32-unknown-unknown	21s
rustfmt nightly on ubuntu	12s
build nightly no-std for wasm32-unknown-unknown	23s
test stable on ubuntu with --features default,serde	50s
clippy nightly on ubuntu	33s
test stable on ubuntu with --no-default-features	39s
test nightly on ubuntu with --features default,serde	47s
test nightly on ubuntu with --no-default-features	43s
	4m 28s

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