Slot-based routing (#573)

test/apps/layout/src/routes/[x]/[y]/[z].svelte

@@ -0,0 +1,18 @@
+<script context="module">
+	import counts from '../_counts.js';
+
+	export function preload() {
+		return {
+			count: counts.z += 1
+		};
+	}
+</script>
+
+<script>
+	import { page } from '@sapper/app';
+
+	export let count;
+</script>
+
+<span>z: {$page.params.z} {count}</span>
+<a href="foo/bar/qux">click me</a>