Introduce `SparseMerkleTree` trait #904

Job	Run time
clippy nightly on ubuntu	46s
rustfmt nightly on ubuntu	14s
build stable no-std for wasm32-unknown-unknown	16s
test stable on ubuntu with --features default,serde	41s
build nightly no-std for wasm32-unknown-unknown	27s
test stable on ubuntu with --no-default-features	45s
test nightly on ubuntu with --features default,serde	48s
test nightly on ubuntu with --no-default-features	43s
	4m 40s

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