Make generateModelBuilder generate deep builders (instead of shallow)

[sav007]

For each builder generate additional setter with Mutator<*.Builder> as an argument:

public Builder hero(@Nonnull Mutator<Hero.Builder> mutator) {
  Hero.Builder builder = this.hero != null ? this.hero.toBuilder() : Hero.builder();

  mutator.accept(builder);

  this.hero = builder.build();
  return this;
}

That will allow user to do next:

data.toBuilder()
  .hero(someHero)
  // or
  .hero(new Consumer<>() {
    void accept(Hero.Builder builder) {
      builder.name("Joe")
       .age(31)
    }
  })

This will be even better with lambda support:

data.toBuilder()
  .hero(someHero)
  // or
  .hero(builder -> builder
       .name("Joe")
       .age(31)
  )

Closes #643

[sav007]

cc: @martinbonnin

[martinbonnin]

Just to let you know I started looking into this but I have troubles importing the snapshot into my project. Somehow the Consumer<> class is not exported not sure exactly why... Okay, rm -rf ~/.gradle and it's working :)

Also it would help to have an integration test where we could demonstrate the usage of this. Not really sure how to do this without adding another submodule as it requires changing some compiler flags.

Anyways, I'll post again when I get it working

[martinbonnin]

Did you use Consumer on purpose ? Why not call the class Mutator or BuilderMutator so as to avoid confusion with the other Consumers (reactivex, java.util, com.annimon.stream, maybe others ?)

[sav007]

Just copied from Java8, but yeah we can rename it