Refactor FunctionLoader so that it returns a cloned function each time

[ejizba]

Related to #553, people will be able to mess with the function more easily, so I want to make sure they can't affect the function across invocations.

Interestingly, we were already cloning the function in getEntryPoint as a part of setting the this arg. I just moved the logic so that it happens during getFunc instead of load and used the slightly cleaner bind instead of apply.

[hossam-nasr]

Seems like some magic to me with the bind business. How were we setting this appropriately before without using bind? Regardless, I don't want to block on this :)

[ejizba]

Before

return function () { return f.apply(obj, arguments); };

After

return loadedFunction.func.bind(loadedFunction.thisArg);

If I were to use the old names, it would look like this:

return f.bind(obj);

The main benefit of bind is you can get rid of the wrapper function() {} stuff. Otherwise, I feel like bind and apply are both pretty magic-y so I don't think it makes much of a difference.