test(tests/playground/ref2): small example using [init]

tests/playground/ref2.lean

@@ -0,0 +1,13 @@
+def init_x : io (io.ref nat) :=
+io.mk_ref 0
+
+@[init init_x] constant x : io.ref nat := default _
+
+def inc : io unit :=
+do v ← x.read,
+   x.write (v+1),
+   io.println (">> " ++ to_string v)
+
+def main (xs : list string) : io unit :=
+do let n := xs.head.to_nat,
+   n.mrepeat (λ _, inc)