forward_lines
Description:
Moves count lines forward, if possible (if count would move past the start or end of the buffer, moves to the start 
      or end of the buffer).
 The return value indicates whether the iterator moved onto a dereferenceable position; if the iterator didn’t move, or moved onto the end 
      iterator, then false is returned. If count is 0, the function does nothing and returns 
      false. If count is negative, moves backward by 0 - count lines.
Parameters:
| this | 
           a TextIter  | 
      
| count | 
           number of lines to move forward  | 
      
Returns:
| 
           whether this moved and is dereferenceable  |