Introduce `SparseMerkleTree` trait #861

Job	Run time
rustfmt nightly on ubuntu	15s
clippy nightly on ubuntu	50s
test stable on ubuntu with --features default,serde	41s
test stable on ubuntu with --no-default-features	42s
build stable no-std for wasm32-unknown-unknown	22s
test nightly on ubuntu with --features default,serde	47s
build nightly no-std for wasm32-unknown-unknown	27s
test nightly on ubuntu with --no-default-features	43s
	4m 47s

Provide feedback