tweak: implement double as test

Author: kanersps

modules/double.lp

@@ -0,0 +1,3 @@
+export fun(x) {
+  return x * 2
+}

std.lp

@@ -1,7 +1,9 @@
 import "./modules/for-loop.lp" as for
 import "./modules/print.lp" as println
+import "./modules/double.lp" as double
 
 export {
   "for": for,
-  "println": println
+  "println": println,
+  "double": double
 }