generate .from signature for intermediate interfaces.

[prior]

Given:

public interface A {
  int getAValue();
}

public interface B {
  int getBValue();
}

public interface AB extends A, B { }

@Value.Immutable
public interface ImmutableABIF extends AB { }

public class ABThing implements AB { ... }

only these from methods are generated:

  public final Builder from(B instance)
  public final Builder from(A instance)
  public final Builder from(ImmutableABIF instance)

I'd really love for

public final Builder from(AB instance)

to also be generated. I can workaround now with this:

// given an ABThing abThing;
ImmutableAB.from((A) abThing).build();

This will actually build all the fields of AB because the private from method actually checks against each possible interface with fields, but I strangely have to convert it to A (or B) to make that work. We should just offer another .from signature to handle this -- right?