Add experimental support for incremental annotation processing in mini-processor

[adriangl]

PR's key points

The PR adds experimental support for incremental annotation processing compilation while using mini-processor

This is done making the annotation processor an Aggregating annotation processor. You can read more about this in Gradle's official documentation here: https://docs.gradle.org/current/userguide/java_plugin.html#sec:incremental_annotation_processing.

You can see some implementations of such annotation processors in the links provided in MiniProcessor.

I also rewrote some code from the processors to make it a bit easier to understand

How to review this PR?

Basically test if builds are faster with the version in this branch and the tips mentioned in the README.md. I left the PR as a Draft until we can verify that this indeed improves build speed

Additional notes

You can find some examples of incremental annotation processors implementations here:

• Aggregating incremental annotation processors implementations:
  • Make annotation processor aggregating/incremental greenrobot/EventBus#617
• Isolating incremental annotation processors implementations (it does not apply here, but they're interesting to read nonetheless):
  • Support gradle incremental processing in code gen square/moshi#824
  • Add support for incremental annotation processing JakeWharton/butterknife#1546