function Fold(params) {
    Transform.call(this, [], params);
  }
Folds one more tuple fields into multiple tuples in which the field
name and values are available under new 'key' and 'value' fields.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.fields - An array of field accessors
  for the tuple fields that should be folded.
@param {Array<string>} [params.as] - Output field names for folded key
  and value fields, defaults to ['key', 'value'].