feat: add sample gcd

gcd.ml

@@ -0,0 +1,18 @@
+(* File gcd.ml *)
+let rec gcd a b =
+  if b = 0 then a
+  else gcd b (a mod b);;
+
+let main () =
+  let len = Array.length Sys.argv in
+  Printf.printf "array length %d\n" len;
+  if len < 3 then (
+    Printf.printf "must have 2 args";
+    exit 0;
+  );
+  let a = int_of_string Sys.argv.(1) in
+  let b = int_of_string Sys.argv.(2) in
+  Printf.printf "gcd of %d and %d is %d\n" a b (gcd a b);
+  exit 0;;
+main ();;
+