[RFC] Nested routes

For nested routes to be fully supported, we'll need a component for each UI library, but we can provide primitives to make implementation easy and share the common logic between them.

- `getMatchedRoutes` \
This returns an array of `Route` objects, ordered parent-first.
Its result is injected before each page starts rendering.
The `Route` component can use this to know if it should render.

- `renderRoute` \
Given a `Route` object, it will load its route props and route module.
Then the route's layout (see #70) has its `render` method called, but with `options.nested` set to true (which signals to the layout that it should produce a result supported by its associated UI library, rather than plain HTML).

So the expected flow goes like this:
- developer uses `Route` component specific to UI library of choice
- component checks if any routes returned by `getMatchedRoutes` have a route pattern identical to its own
- component calls `renderRoute` (if it finds a matching route) and does whatever with the result

Note that nested routes still need to be declared within the routes module, because that's where the route props, module, layout, and more are defined. That declaration is also used when pre-rendering and (eventually) bundling.

### What's so great about nested routes?

Nested routes let you tell Saus that certain route props don't need to delay rendering until a certain point in the component tree. This means you can stream HTML to the browser before all data is loaded, which can vastly improve TTFP (time to first paint) and therefore make your site feel faster.