Puzzle: Read Preserving Comments

Here is a little Lisp puzzle for you:

I want to read a Lisp file and produce a list of the top-level forms in the file, but I want to preserve comments. Any comment that appears on a line within a top-level form should be associated with that form. Comments that appear by themselves at top level should be associated with the following top-level form. For example, if the file contains:

;;; -*- Lisp -*-

;; This file is a test
  
(in-package "CL-USER")

;; This is the test function:
(defun foo (arg)
  ;; This function does nothing
  (declare (ignore arg))
  nil)    ;; Return nil

(defparameter *x* 42)
(defparameter *y* 99)  ;; This is y

;; The BAR function doesn't do anything either

(defun bar ()  
  nil)

;; Trailing comment in file.

Then I want to produce the following list:

'(";;; -*- Lisp -*-

;; This file is a test
  
(in-package \"CL-USER\")
"
 "
;; This is the test function:
(defun foo (arg)
  ;; This function does nothing
  (declare (ignore arg))
  nil)    ;; Return nil
"
 "
(defparameter *x* 42)
"
 "(defparameter *y* 99)  ;; This is y
"
 "
;; The BAR function doesn't do anything either

(defun bar ()  
  nil)
"
 "
;; Trailing comment in file.
")

This puzzle is not difficult, but it is tricky to get the edge cases right.