Support Svelte 3

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

runtime/src/app/prefetch/index.ts

@@ -0,0 +1,14 @@
+import { select_target, prefetching, set_prefetching, hydrate_target } from '../app';
+import { Target } from '../types';
+
+export default function prefetch(href: string) {
+	const target: Target = select_target(new URL(href, document.baseURI));
+
+	if (target) {
+		if (!prefetching || href !== prefetching.href) {
+			set_prefetching(href, hydrate_target(target));
+		}
+
+		return prefetching.promise;
+	}
+}