dev: improve rlp::decode() gas consumption

[Quentash]

Since #457, the gas consumption of rlp::decode() has increased by 1-2% for failing tests and by 9-11% for succeeding tests. The PR was merged because we needed the ability to recursively decode transaction bytes, but there may be room for gas optimization.
Please study the possibility of reducing rlp::decode() gas consumption and implement the eventual solution.