code: @ApiFeature(since = VERSION_5_1_0, stability = STABLE)
      public static <Happy, Sad, Field> FieldValidator<Happy, Sad, Field> fieldValidator(Mapping<Happy, Field> fieldExtractor, Validator<Field, Sad> fieldValidator) {
          return new FieldValidator<>(fieldExtractor, fieldValidator);
      }
docs: Produces a {@link FieldValidator} that will validate the extracted field using the given validator.
  
  @param fieldExtractor Extracts the {@link Field} from the {@link Happy}
  @param fieldValidator The {@link Validator} will validate the {@link Field}, possibly producing a {@link Sad} failure
  @param <Happy>        The type that contains the {@link Field}
  @param <Sad>          The type of validation failure
  @param <Field>        The type of the field to validate
  @return The validator