Optimize RLP serialization

[chirag-parmar]

• Implement two pass algorithm
• Write a benchmarking test comparing two pass with the previous method
• Implement rlpHash by combining two pass algorithm with chunked updates
• Wriite a benchmarking test comparing the new rlpHash with the old one
• use a double buffer technique for hashing so that we don't hit the branching condition in keccak.update for single byte updates