Add MerkleMountainRangeRoot digest item in header (#1)

* add new merkle mountain range digest item for header

* introduce merkle mountain range lib

* add mmr root to digest in System::finalize(), waiting mmr crate supporting T:Hash to make compile pass

* upgrade to use mmr lib from ckb

* move mmr logic to seprate darwinia pallet

* remove useless lines

* add tests for header mmr digest

* update test comment