Added (list-set!)

TODO

@@ -1,8 +1,5 @@
 Working TODO list:
 
- - Add (list-set!) from r7rs
-   will likely require tweaks to GC similar to (set-car! / set-cdr!)
-
  - Error handling
    need to perform much more error handling of input code. one of the biggest is to report if a function is passed the wrong number of arguments, as this will result in segfauls, bad transport errors, etc downstream if it is allowed.
 

trans.scm

@@ -117,6 +117,9 @@
       lst
       (list-tail (cdr lst) (- k 1))))
   (define (list-ref lst k)  (car (list-tail lst k)))
+  (define (list-set! lst k obj)
+    (let ((kth (list-tail lst k)))
+      (set-car! kth obj)))
   (define (reverse lst)   (foldl cons '() lst))
   (define (symbol=? sym1 sym2 . syms)
     (and