This entirely abolishes the "global scope object" in AVM2. I even had to redefine several global object functions to work with the bottom of the scope stack, which seems to be where ASC likes to stick the script scope.
This code also ensures that the prototypes of each system object are created in the appropriate `TObject` impl. This ensures that, for example, `new Array` hands you back an actual array.
This has some particularly annoying consequences for initialization order: notably, we can't actually create any ES4 classes using the standard machinery until after the three objects I just mentioned get created. Ergo, we have to create them through lower-level means, handing prototypes around, and then initialize AVM2's system prototypes list for it.
When we start adding more system prototypes, we'll also have to fill the extras with blank objects and then slot them in as we create them.
This was surprisingly tricky - due to the need to look up superclasses, class trait instantiation requires an active `Activation` and `UpdateContext`. We can't get those during VM instance creation, since the player needs the VM first before it can give it a context to work with. Ergo, we have to tear the global scope initialization in two. At the first possible moment, the player calls a new `load_player_globals` method that initializes all class traits in global scope.
This code is slightly over/under-precise compared to AVM2. This is because we handle precision limiting in binary floats rather than as part of the float printing process. Flash Player may also be rounding differently than us. However, I'm pretty sure ECMA-262 allows us to slightly differ here.
This also results in a far reduced role for `ReturnValue`, since I also took the liberty of removing most of it's use. Furthermore, I also made it apply equally to native and AVM2 code, which ensures all native implementations of methods don't double-borrow.
In AVM1, `ReturnValue` was actually removed entirely, because it's not needed. I attempted to do the same, but the fact that we're currently embedding `ScriptObjectData` in native objects means that we need it for virtual properties. Otherwise, virtual property implementations will see locked objects, which is bad.
We still reuse the `FunctionObject` machinery internally. If necessary, we may want to split this into a separate `ClassObject` if some internal `TObject` method needs replacing for classes.
Also, implement a method table that method traits can optionally add themselves to.
Also also, add the ability to invoke a method without a `this` object. This required a non-trivial refactoring of the activation machinery, and changes to the signature of `NativeFunction`, and all native AVM2 functions.
David Wendt