Support MMR digest in block header

[hackfisher]

Closes #352
Closes #344

This is to support and include MMR in darwinia block header, so the darwinia's light client and chain relay can use this mmr value to build super light client.

Blockchain history digest item that contains the merkle mountain range root at given block. It is created for providing super light client a commitment of all the previous blocks.

• Introduce mmr lib and update_logs in header (System::on_finalize)
• Update mmr lib refer:
  • https://github.com/tari-project/tari/tree/development/base_layer/mmr
  • https://github.com/nervosnetwork/merkle-mountain-range
• Replace blake2 with T::Hashing::hash_of
• Refactor concern: Move deposit digest log from System to Darwinia's Module
• Does it require special handling in Genesis block? (What's genesis block's parent hash)
• Won't support a. merge hash with index? b. detail in push c. with total difficulty
• Get Proof RPC Call and verify function for testing. (verify rpc might not need since it should happen on other chains.)
• Tests
• Review append only ModuleMMRStore and checking the logic
• Document
• Update substrate dependency

[AurevoirXavier]

Looks ok, let's give it a try

[yanganto]

Although there are only two workers in Travis free plan, it still can run concurrently with the same stage name.