* Adjust ProGuard default rules and shrinking tests
* Adjust comment
* Add shrinking test for class without no-args constructor; improve docs
* Improve Unsafe mention in Troubleshooting Guide
* Improve comment for `-if class *`
* Add code shrinking tools integration test
* Keep no-args constructor of classes usable with JsonAdapter
* Add library ProGuard rules for Gson
They are automatically applied for all users of Gson, see
https://developer.android.com/build/shrink-code#configuration-files
* Skip japicmp-maven-plugin for shrinker-test
* Add more tests for JsonAdapter, add tests for generic classes
* Extend default constructor test
* Add Troubleshooting Guide entry for TypeToken
