constructor(name, params, body) {
    super();
    this.name = name;
    this.params = params;
    this.block = body;
    if ('function' == typeof params) this.fn = params;
  }
Initialize a new `Function` with `name`, `params`, and `body`.

@param {String} name
@param {Params|Function} params
@param {Block} body
@api public