Function:    TimerStr()

               TimerStr() returns the current timer value (as set
               by StartTimer()) in a string in the form
               "HH:MM:SS". GetTimer() does the same but returns a
               Numeric representing the #seconds passed.
               TimerStr() was also written fully in assembly, and
               is thus very compact and efficient.

  Syntax:      TimerStr().

  Arguments:   None.

  Returns:     A string representing Elapsed time in the form
               "HH:MM:SS".

  Usage:       * - See StartTimer().