/*
 *  call-seq:
 *     dvector.where_min         ->  int or nil
 *     dvector.where_first_min   ->  int or nil
 *  
 *  Returns the index of the first entry with the minimum value in _dvector_, <code>nil</code> if
 *  _dvector_ is empty.
 *     
 *     a = Dvector[ 1, 2, -3, 4, -5, 4, 3, -5, 2 ]
 *     a.where_min        -> 4
 *     Dvector[].where_min   -> nil
 */ 
VALUE dvector_where_min(VALUE ary) {