Note that host objects that do so will *not* have access to their standard representation from within member functions - you will need to extend the interface to accomodate for them. This is due to long-standing limitations with type IDs and downcasting with types that bear lifetimes - it's entirely an unsafe operation and exposing such a facility to safe Rust is unsound. However, this will at least let us separate out several things from ScriptObject that don't need to be there for the time being.
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.
Mike Welsh