Add rna transcription exercise

[mbertheau]

No description provided.

[lexi-lambda]

Use in-string to iterate through the string. Also, use case instead of cond for comparing against datums.

(define (to-rna sequence)
  (list->string
   (for/list ([c (in-string sequence)])
     (case c
       [(#\G) #\C]
       [(#\C) #\G]
       [(#\T) #\A]
       [(#\A) #\U]))))

(Also, I fixed the indentation a little.)

[mbertheau]

All done :)