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

typings/packer/base.d.ts

@@ -0,0 +1,7 @@
+import { eventWithTime } from '../types';
+export declare type PackFn = (event: eventWithTime) => string;
+export declare type UnpackFn = (raw: string) => eventWithTime;
+export declare type eventWithTimeAndPacker = eventWithTime & {
+    v: string;
+};
+export declare const MARK = "v1";