Add rna transcription exercise

config.json

@@ -4,6 +4,7 @@
   "repository": "https://github.com/exercism/xracket",
   "active": false,
   "problems": [
-    "bob"
+    "bob",
+    "rna-transcription"
   ]
 }

rna-transcription/example.rkt

@@ -0,0 +1,12 @@
+#lang racket
+
+(define (to-rna sequence)
+  (list->string
+    (for/list ([c (in-string sequence)])
+      (case c
+        [(#\G) #\C]
+        [(#\C) #\G]
+        [(#\T) #\A]
+        [(#\A) #\U]))))
+
+(provide to-rna)

rna-transcription/rna-transcription-test.rkt

@@ -0,0 +1,16 @@
+#lang racket
+
+(require rackunit
+         rackunit/text-ui
+        "rna-transcription.rkt")
+
+(define suite
+  (test-suite
+    "Tests for the rna-transcription exercise"
+    (check-equal? (to-rna "G") "C" "transcribes guanine to cytosine")
+    (check-equal? (to-rna "C") "G" "transcribes cytosine to guanine")
+    (check-equal? (to-rna "T") "A" "transcribes thymine to adenine")
+    (check-equal? (to-rna "A") "U" "transcribes adenine to uracil")
+    (check-equal? (to-rna "ACGTGGTCTTAA") "UGCACCAGAAUU" "transcribes all occurences")))
+
+(exit (if (zero? (run-tests suite)) 0 1))