Support and include MMR in blockheader

[hackfisher]

For easier darwinia relay support

[aurexav]

Transfer to https://github.com/darwinia-network/research?

[hackfisher]

Transfer to https://github.com/darwinia-network/research?

Actually the research is almost done? The works left is about the engineering stuffs related to Substrate Runtime primitives, constructing block and MMR calculating.

Here are some code links:

Header Struct:
https://github.com/paritytech/substrate/blob/v2.0.0-alpha.3/primitives/runtime/src/generic/header.rs#L39

Block Builder:
https://github.com/paritytech/substrate/blob/v2.0.0-alpha.3/client/block-builder/src/lib.rs#L205

MMR struct and append:

https://github.com/darwinia-network/darwinia/blob/icefrog/core/merkle-mountain-range/src/mmr.rs#L44

It will be great if we can do it in Crab Network.

[hackfisher]

We could implement it in a separate branch while waiting for the substrate stable release.

[hackfisher]

There is a digest data in Header struct, could we make use of it for easy extend of Substrate code, if it works, then we might not need to introduce modification to Substrate runtime primitives too much.

https://github.com/paritytech/substrate/blob/v2.0.0-alpha.3/primitives/runtime/src/generic/header.rs#L52

pub struct Header<Number: Copy + Into<U256> + TryFrom<U256>, Hash: HashT> {
	/// The parent hash.
	pub parent_hash: Hash::Output,
	/// The block number.
	#[cfg_attr(feature = "std", serde(
		serialize_with = "serialize_number",
		deserialize_with = "deserialize_number"))]
	pub number: Number,
	/// The state trie merkle root
	pub state_root: Hash::Output,
	/// The merkle root of the extrinsics.
	pub extrinsics_root: Hash::Output,
	/// A chain-specific digest of data useful for light clients or referencing auxiliary data.
	pub digest: Digest<Hash::Output>,
}