Make `Context` not generic, and reference it in `CoreInner`

Instead of using AsRef<LoopRef> + Clone, this uses a new AsLoop trait, which can be used to get a reference to the Rc of the inner type, that can be converted to a trait object for dynamic dispatch.

Referencing Context in CoreInner fixes #64 and #76.

The trait definition here is a little unusual, but I don't know if there's a better way to do this. Dynamic dispatch of some form or another is probably the correct thing to do.