Yew: Rename Switch trait

[hgzimmerman]

Description

While Switch is an ok name for what it does, I think something along the lines of Routable would be more appropriate.

I don't intend to make this change for 8.0, but would like to create this issue so that interested parties can prepare their codebase for this change.

---

This change will come in 3 pieces.

1. Add Routable alias (0.8.0)
2. Rewrite macro & Trait to use Routable instead, deprecate Switch, make Switch an alias to Routable (0.9.0)
3. Remove Switch (0.10.0)

[Boscop]

@hgzimmerman Also, I think it would make much more sense to name the to attribute at instead:

#[derive(Switch, Debug)]
pub enum AppRoute {
    #[at = "/profile/{id}"]
    Profile(u32),
    #[at = "/forum{*:rest}"]
    Forum(ForumRoute),
    #[at = "/"]
    Index,
}

#[derive(Switch, Debug)]
pub enum ForumRoute {
    #[at = "/{subforum}/{thread_slug}"]
    SubForumAndThread{subforum: String, thread_slug: String}
    #[at = "/{subforum}"]
    SubForum{subforum: String}
}

Because links/buttons route to somewhere (it's an action to follow a link) but the routes are defined at a certain url, and at also reflects the fact that it's declarative better.