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/index.d.ts

@@ -0,0 +1,2 @@
+export { pack } from './pack';
+export { unpack } from './unpack';