Use custom JSON marshal instead of object duplication

Currently, some of the Transaction-related objects have two forms, which implement normal type and for marshaling.

https://github.com/CodeChain-io/codechain-sdk-go/blob/6ad3e76c4dfba22b0867764b8c67500dce7518d2/core/transaction/signedTransaction.go#L11-L17
https://github.com/CodeChain-io/codechain-sdk-go/blob/6ad3e76c4dfba22b0867764b8c67500dce7518d2/core/transaction/signedTransaction.go#L19-L25

This implementation is from [codechain-sdk-js](https://github.com/CodeChain-io/codechain-sdk-js).
But since golang can implement custom marshaling function ([link1](http://choly.ca/post/go-json-marshalling/
)) ([link2](https://play.golang.org/p/CTmJhTiGAM)), I think duplication of the objects is unnecessary.

Start with core/transaction/signedTransaction.go, especially the linked lines showing the normal and marshaling-specific forms. Compare the duplicated Transaction-related objects and review Go's custom JSON marshaling approach. Done means the duplicate object forms are replaced where appropriate while preserving the existing JSON output.

由索引模型根据 Issue 内容生成。