Refactor the Runtime

[matthunz]

• Only enter the Runtime once per frontend application.
• Creates RuntimeState and redefines Runtime as a wrapper around it. This lets us simplify the external API without requiring users know how the Runtime is stored (similar to Tokio).

pub(crate) struct RuntimeState { ... }

#[derive(Clone)]
pub struct Runtime {
    pub(crate) state: Rc<RuntimeState>,
}

• The current Runtime seems to use a thread-local stack of runtimes with a Vec. This replaces the stack with a single Option and adds a new Runtime::take method (to exit the current runtime and return it to the user, keeping that same behavior of a stack if desired)
• Investigate fullstack suspense