Requisites for EIP-2718 application

[trinhdn2]

These PRs include more flexible changes in RLP library and some code refactoring. These are necessary to resolve issue #355 by PR #380, and also deliver a more up-to-date codebase for further critical features and changes.

• [Non-breaking changes] Update RLP library and RLP encoder code generation tool for structs #358
• Enhance return type for Execution #366
• [Non-breaking changes] Move Message struct from types to core package #367
• [Non-breaking changes] Optimize receipt storing #368
• [Non-breaking changes] Unify multiple keccakState interfaces #369
• [Non-breaking changes] Move statedb.Account struct to types.StateAccount #373
• [Non-breaking changes] Move database_util.go to rawdb package #374