简单的 LOOP 形式

没有特殊关键字的简单 LOOP 表单：

(loop forms...)

为了摆脱循环，我们可以使用 (return <return value>)`

一些例子：

(loop (format t "Hello~%"))  ; prints "Hello" forever
(loop (print (eval (read)))) ; your very own REPL
(loop (let ((r (read)))
        (typecase r
         (number (return (print (* r r))))
         (otherwise (format t "Not a number!~%")))))