Support defun mechanism

[dakusui]

, where if you define a function print_twice by

{
    "define": {
	"print_twice": ["sequential",
		     ["print", [0]],
		     ["print", [0]]
		    ]
	}
    }
}

, you can call it by

["print_twice", "Hello, world"]

, giving

Hello, world
Hello, world