exporter = function (KEY, exec, ALIAS) {
  var exp = {};
  var FORCE = fails(function () {
    return !!spaces[KEY]() || non[KEY]() != non;
  });
  var fn = exp[KEY] = FORCE ? exec(trim) : spaces[KEY];
  if (ALIAS) exp[ALIAS] = fn;
  $export($export.P + $export.F * FORCE, 'String', exp);
}
the minimum amount of gas the tx must have (DataFee + TxFee + Creation Fee)
@return {BN}