Introduce `SparseMerkleTree` trait #906

Job	Run time
rustfmt nightly on ubuntu	13s
clippy nightly on ubuntu	50s
test stable on ubuntu with --features default,serde	41s
build stable no-std for wasm32-unknown-unknown	16s
test stable on ubuntu with --no-default-features	43s
build nightly no-std for wasm32-unknown-unknown	22s
test nightly on ubuntu with --features default,serde	48s
test nightly on ubuntu with --no-default-features	49s
	4m 42s

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