Notably, all of the `Avm1` "run stack frame" functions can no longer take a self parameter as the update context they will be getting also has that same parameter. Ergo, they're associated functions that get the moral equivalent of self from the update context.
This also introduces a new `Activation::from_stub` which creates a stub frame that runs everything on the main movie in layer 0. This significantly reduces boilerplate code elsewhere in the project.
The process of constructing an `Activation` now involves calling `UpdateContext.reborrow`, which "sheds" a lifetime by copying all of the borrows into a new "owned" context with that lifetime.
Likewise, to call out to functions that don't need an `Activation`, just borrow the context out of the current activation. You can also construct child-frame activations by reborrowing the parent activation's context.
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