Use custom JSON marshal instead of object duplication

[cubismic]

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

codechain-sdk-go/core/transaction/signedTransaction.go

Lines 11 to 17 in 6ad3e76

	type SignedTransactionJSON struct {
	BlockNumber int `json:"blockNumber,omitempty"`
	BlockHash string `json:"blockHash,omitempty"`
	TransactionIndex int `json:"transactionIndex,omitempty"`
	Sig string `json:"sig"`
	Hash string `json:"hash"`
	}

codechain-sdk-go/core/transaction/signedTransaction.go

Lines 19 to 25 in 6ad3e76

	type SignedTransaction struct {
	Unsigned TransactionInterface
	BlockNumber *int
	BlockHash *primitives.H256
	TransactionIndex *int
	signature []byte
	}

This implementation is from codechain-sdk-js.
But since golang can implement custom marshaling function (link1) (link2), I think duplication of the objects is unnecessary.

[cubismic]

This is related to #53

[cubismic]

#63