This type explicitly signals if an immediate value is to be returned, if a value is to be returned on the stack, or if no return value is to be generated. Holders of a `ReturnValue` can also use `and_then` to schedule a `StackContinuation` to be executed when and if that value is ready.
`StackContinuations` now yield `ReturnValues` as well, so they have a moderate level of composability. For example, if you need to get a property from an object and push it on the stack, you can return the result of calling `get` directly and the machinery ensures it eventually gets there.
This involved yet another macro, `and_then!`, to avoid a ridiculous amount of duplicate code. It calls a continuation whenever it's value is ready, even if the value resolved on the Rust stack.
`locals_into_form_values` does not currently support this. It skips any property that does not resolve on the Rust stack. Future work is required to resolve 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)