This has the side effect of letting us remove the `Option` on register_count since setting this to `0` is equivalent now. Furthermore, we can skip an allocation if a function requests no registers.
On SWF5, the SWF version of the callee depends on it's this parameter. Calling it as a function rather than a method downgrades the callee. SWF6+ use the callee's inherent SWF version and do not allow changing the SWF version like this.
* Remove clone calls from Copy objects
* Used Iterator::cloned() instead of manually cloning
* Pass swf::Function into AvmFunction2::new()
* Use action_clone_sprite
1. We no longer implicitly return Undefined unless we're specifically returning from a function (this also keeps actions from filling the stack with Undefined)
2. With scopes are now always inserted behind the current set of locals rather than overriding them
3. `ActionSubtract` now subtracts (instead of adds)
Mike Welsh