seek method

@override void seek(int _index)

consume() ahead until p==_index; can't just set p=_index as we must update line and column. If we seek backwards, just set p

Implementation

@override
void seek(int _index) {
  if (_index <= this._index) {
    this._index = _index; // just jump; don't update stream state (line,
    // ...)
    return;
  }
  // seek forward
  this._index = min(_index, size);
}