Slot-based routing (#573)

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

@@ -0,0 +1,21 @@
+<script context="module">
+	import counts from '../_counts.js';
+
+	export function preload() {
+		return {
+			count: counts.y += 1
+		};
+	}
+</script>
+
+<script>
+	import { page } from '@sapper/app';
+
+	export let count;
+	export let segment;
+</script>
+
+<span>y: {$page.params.y} {count}</span>
+<slot></slot>
+
+<span>child segment: {segment}</span>