Introduce `SparseMerkleTree` trait #885

Job	Run time
rustfmt nightly on ubuntu	13s
clippy nightly on ubuntu	35s
build stable no-std for wasm32-unknown-unknown	21s
test stable on ubuntu with --features default,serde	39s
build nightly no-std for wasm32-unknown-unknown	26s
test stable on ubuntu with --no-default-features	48s
test nightly on ubuntu with --features default,serde	52s
test nightly on ubuntu with --no-default-features	45s
	4m 39s

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