add coroutine

test_scripts/co.lua

@@ -0,0 +1,13 @@
+local function sleep(n)
+  os.execute("sleep " .. tonumber(n))
+end
+
+local co = coroutine.create(function(x)
+  sleep(3)
+  print("co -> ", x)
+  end)
+
+coroutine.resume(co, 42)
+
+ngx.say("main")
+