Save as zlib-compressed data

handles https://github.com/google/stagehand/issues/45

Very simply tweak data as compressed by using `ZLIB.encode`.

`webapp_data.dart` is 215k -> 65k.

If we compress the whole `data` from `_data.dart`, then we might able to have better performance as I think.
But If we compresse the whole data, we're not able to catch what file exist in `_data.dart`.

@devoncarew for review and wisdom.