Closures in inception need improvement

[skx]

At the moment there is support for lambdas and closures in inception so this works:

(defun makeAdder (n)
  (lambda (x)
   (+ x n)))

(defun main ()
  (let ((ten (makeAdder 10)))
  (println (ten 15))
  (println (ten 25))
  ))

However the captured values are essentially read-only, so this example fails:

 (defun counter ()
   (let ((n 0))
     (lambda ()
       (set! n (+ n 1))
      n)))

 (defun main()
   (let ((f (counter)))
     (println (f))
     (println (f))))

I think this should be an easy fix, since we're passing around env everywhere. While we cannot have a closure modify a single value in the parent scope we do have the ability to change things in-place for lists, via nth! and our closure is itself a list:

(defun closure (params body env)
(list "closure" params body env))

When the closure runs it should mutate its own env, and that would be enough?