Use plain for loop instead of for-in loop to iterate the Array elements.

Use plain for loop instead of for-in loop to iterate the Array elements to avoid js errors if someone had attached extra methods to the Array.prototype.

For example:
If someone integrates skulpt with some other js library which had some code like these:
Array.prototype.firstOrDefault = function(){...}

Then there would be javascript errors thrown out when executing:
for (var tix in TurtleGraphics.turtleList) {...}