Packer (#172)

* introduce pako and add general packer interface

* add tests for packer

* use function API instead of class API for better tree shaking support

* refcatoring the rollup bundle config

src/packer/base.ts

@@ -0,0 +1,10 @@
+import { eventWithTime } from '../types';
+
+export type PackFn = (event: eventWithTime) => string;
+export type UnpackFn = (raw: string) => eventWithTime;
+
+export type eventWithTimeAndPacker = eventWithTime & {
+  v: string;
+};
+
+export const MARK = 'v1';