Investigate possiblility of removing inner Arc in MessageContent.

[rsimmonsjr]

This is an issue for efficiency and simplification. Currently the MessageContent type looks like the following:

pub enum MessageContent {
    /// The message is a local message.
    Local(Arc<dyn ActorMessage + 'static>),
    /// The message is from remote and has the given hash of a [`std::any::TypeId`] and the
    /// serialized content.
    Remote(Vec<u8>),
}

Note that the local message is holding the content inside an inner Arc. It would be much nicer if we could get rid of the inner Arc if possible because it would reduce complexity and indirection.

[rsimmonsjr]

Actually it is not possible because of Sized constraints in rust. The type dyn ActorMessage has no size and that messes up a lot of things.