Introduce `SparseMerkleTree` trait #894

Job	Run time
clippy nightly on ubuntu	36s
build stable no-std for wasm32-unknown-unknown	20s
test stable on ubuntu with --features default,serde	41s
build nightly no-std for wasm32-unknown-unknown	26s
rustfmt nightly on ubuntu	13s
test stable on ubuntu with --no-default-features	41s
test nightly on ubuntu with --features default,serde	47s
test nightly on ubuntu with --no-default-features	47s
	4m 31s

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