Implement 'string-set!' in Scheme

[Katie Burke]

string-set! string-set! changes the nth element of string to char.
http://www.scheme.com/tspl4/objects.html#./objects:s221
This example should return "hi there".

(let ([str "hi three"])
  (string-set! str 5 #\e)
  (string-set! str 6 #\r)
  str)

Traceback (most recent call last):
File "stdin", line 1, col 1, calling 'unknown'
File "stdin", line 2, col 4
RunTimeError: unbound variable 'string-set!'