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/pack.ts

@@ -0,0 +1,10 @@
+import { deflate } from 'pako/dist/pako_deflate';
+import { PackFn, MARK, eventWithTimeAndPacker } from './base';
+
+export const pack: PackFn = (event) => {
+  const _e: eventWithTimeAndPacker = {
+    ...event,
+    v: MARK,
+  };
+  return deflate(JSON.stringify(_e), { to: 'string' });
+};