Introduce `SparseMerkleTree` trait #895

Job	Run time
rustfmt nightly on ubuntu	14s
build stable no-std for wasm32-unknown-unknown	21s
test stable on ubuntu with --features default,serde	47s
clippy nightly on ubuntu	31s
test stable on ubuntu with --no-default-features	38s
test nightly on ubuntu with --features default,serde	58s
build nightly no-std for wasm32-unknown-unknown	22s
test nightly on ubuntu with --no-default-features	45s
	4m 36s

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