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}/>