make TXer and Payload implement Serializable

Both are duplicating Serializable at the moment, but let's keep them for the
future.

pkg/core/transaction/txer.go

@@ -5,6 +5,5 @@ import "github.com/CityOfZion/neo-go/pkg/io"
 // TXer is interface that can act as the underlying data of
 // a transaction.
 type TXer interface {
-	DecodeBinary(*io.BinReader) error
-	EncodeBinary(*io.BinWriter) error
+	io.Serializable
 }

pkg/network/payload/payload.go

@@ -4,8 +4,7 @@ import "github.com/CityOfZion/neo-go/pkg/io"
 
 // Payload is anything that can be binary encoded/decoded.
 type Payload interface {
-	EncodeBinary(*io.BinWriter) error
-	DecodeBinary(*io.BinReader) error
+	io.Serializable
 }
 
 // NullPayload is a dummy payload with no fields.