XPtimerStr()
  Return the elapsed time in a time string
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  Function:    XPtimerStr()

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

  Syntax:      XPtimerStr().

  Arguments:   None.

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

  Usage:       * - See XPstartTimer().