Support Svelte 3

fixes #546, #551, #552, #554

runtime/internal/Sapper.html

@@ -0,0 +1,12 @@
+<script>
+	import { setContext } from 'svelte';
+	import { CONTEXT_KEY } from './shared';
+
+	export let Root;
+	export let props;
+	export let session;
+
+	setContext(CONTEXT_KEY, session);
+</script>
+
+<Root {...props}/>

runtime/internal/layout.html

@@ -0,0 +1 @@
+<svelte:component this={child.component} {...child.props}/>

runtime/internal/shared.mjs

@@ -0,0 +1,10 @@
+import { writable } from 'svelte/store';
+
+export const stores = {
+	preloading: writable(false),
+	page: writable(null)
+};
+
+export const CONTEXT_KEY = {};
+
+export const preload = () => ({});