Update example output comments

Add `Output` comments to the example files to reflect the actual output
of the programs. This helps with verifying correctness and understanding
example behavior.

examples/optimizer_purity.myc

@@ -1,5 +1,6 @@
 ;; Benchmark: 92ns
 ;; Benchmark-Repeat: 21507
+;; Output: 31.41592653589793
 (do
   (def area (fn [x] (* PI x)))
   (area 10)