Scoped messages

[TylerBloom]

Currently, all messages going into an actor must have a static lifetime. While this can be managed by cloning, it would be nice to provide an API similar to that of thread::scope. This is conceptually very easy, but there are a lot of cornercases, and expressing it will be difficult.

Simply managing the lifetimes of this interprocess communication will require a fair amount of consideration (and a manual Drop implementation). But beyond the Rust-based issues, rendering this pattern into the current semantics of troupe will also be difficult. How do you open a scope? How does the scheduler/actor handle this? Does this require a second actor state? Maybe a trait for the messages?