Added two new functions for cleaning up arrays. 

I could have sworn flixel core would have had utility functions for cleaning up arrays, but I can't find them anywhere, not in FlxDestroyUtil or in FlxArrayUtil. 

At any rate, here's two useful functions for cleaning up arrays. 

destroyArray() will go through an Array<IFlxDestroyable> and call FlxDestroyUtil.destroy() on everything and also clear out the array structure.

clearArray() will just clear the array structure of an Array<Dynamic>, but leave all the object data alone. 

I use my own home-grown versions of these all the time, seems like it would be useful to have in flixel-core, especially given the new utility class. 

Do these make more sense here or in FlxArrayUtil?