Calculate Chunk Root Using Blob Serialization & Proof of Custody

[rauljordan]

Hey all,

To continue with the work done on #100 and #92 with respect to creating a Shard struct and the blob serialization implementation done by @nisdas, we now need to calculate ChunkRoot according to the serialization algorithm on the body of a collation.

Context

In #100, I included a simple CalculateChunkRoot method on the Collation type. Given this was out of scope of my PR and blocked by #92 being merged, I chose to have the chunk root be a simple, sha3 hash of the collation body. We need to open a PR that uses the blob serialization to modify the following function created in #100:

// CalculateChunkRoot updates the collation header's chunk root based on the body.
func (c *Collation) CalculateChunkRoot() {
	// TODO: this needs to be based on blob serialization.
	// For proof of custody we need to split chunks (body) into chunk + salt and
	// take the merkle root of that.
	chunkRoot := common.BytesToHash(c.body)
	c.header.data.ChunkRoot = &chunkRoot
}

Additionally, for proof of custody (See that ETHResearch post for context), we will need to split the chunks of the body into chunk + salts and take the merkle root of those combined values. This proof of custody mechanism would add more skin-in-the-game for the notarization scheme, preventing them from being lazy when voting on collations.