Improve RLP macros

Our RLP heavily depends on Ethereum's implementation(https://github.com/paritytech/rlp).
They provide some procedural macros that generate encoding/decoding code for basic objects,
but it lacks of many details and functionality.
Though it helps on some level, we had to manually implement encoding/decoding routines in many cases.
I believe that there will be countless new types of data that should be serialized, and we are going to code repetitive functions again and again if there's no some new powerful metaprogramming support.

(see https://github.com/CodeChain-io/foundry/issues/65)

まず、paritytech/rlp にある既存の RLP 実装と Ethereum の手続き型マクロを確認し、その後 CodeChain-io/foundry#65 の関連する議論を読んでください。この issue ではファイル、テスト、または具体的なマクロ設計が特定されていないため、実装前に必要な機能と完了条件を定義する必要があります。

索引モデルが issue の本文から書いたものです。