def append_opcode(opcode)
      opcode = Opcodes.small_int_to_opcode(opcode) if -1 <= opcode && opcode <= 16
      raise ArgumentError, "specified invalid opcode #{opcode}." unless Opcodes.defined?(opcode)
      chunks << opcode.chr
      self
    end
def append_opcode(opcode)
      opcode = Opcodes.small_int_to_opcode(opcode) if -1 <= opcode && opcode <= 16
      raise ArgumentError, "specified invalid opcode #{opcode}." unless Opcodes.defined?(opcode)
      chunks << opcode.chr
      self
    end
append opcode to payload
 @param [Integer] opcode append opcode which defined by Bitcoin::Opcodes
 @return [Script] return self