code: public org.inferred.freebuilder.processor.property.Property.Builder mapName(
        UnaryOperator<String> mapper) {
      Objects.requireNonNull(mapper);
      return setName(mapper.apply(getName()));
    }
docs: Replaces the value to be returned by {@link
  org.inferred.freebuilder.processor.property.Property#getName()} by applying {@code mapper} to
  it and using the result.
  
  @return this {@code Builder} object
  @throws NullPointerException if {@code mapper} is null or returns null
  @throws IllegalStateException if the field has not been set