Use child scopes for pages

[arctic-hen7]

Right now, Perseus renders all user pages under the single, global cx scope, which is used for the entire app. This works well, until you use something like the router state (a global entity) and log every time there's a page change. I tried this in my rebuild of my own blog, and found very quickly that the logs were accumulating: the listeners were not being killed when I was navigating off the page. In other words, all listeners on global events were accumulating in Perseus apps. This may have been the cause of the error I occasionally observed while working on the Perseus website for long stretches, whereby my browser would freeze up after a few hours until I killed the tab. I now believe that may have been due to listener accumulations and improper memory clearing.

This PR adds a RouteManager struct to Perseus, which bundles together a Signal registered on the app scope for storing the current View<G> with a ScopeDisposer for the current view's scope. All user pages are rendered in a child scope (created by the #[template] macro, for now), and, when a new page is rendered, the old view is replaced, and the old scope disposer is disposed of. This introduces a single line of unsafe to the Perseus core, with safety clearly enforced by the macros. (Users who avoid #[template] should be cautious not to dispose of a scope inside that scope, which is pretty much common sense, and very clearly documented.) This entirely prevents the issues explained above.

[arctic-hen7]

Note that the Scope provided to user apps is now a child scope, but, for some reason, the Scope type still works (even though I believe it should be a BoundedScope<'child, 'app> now). I'm not sure why this is, but it doesn't seem to have any adverse effects. @lukechu10, do you have any input on this?

[arctic-hen7]

No clue what has mangled the CI, but tests are all passing locally, so this is ready to be merged.