Part 19: Remove Guava usage from client code

It's still used in some tests. Also I can't really remove it from REST generators because Guava's EventBus is required. It should be possible to remove it when I improve the serialization generation, but this part is not gonna happen anytime soon.

btw, Guava is a great library and I encourage everyone to use it in their app. The less dependencies a framework requires, the better.