/*
 *  call-seq:
 *     dvector.index(number)   ->  int or nil
 *  
 *  Returns the index of the first object in _dvector_  
 *  <code>==</code> to _number_. Returns <code>nil</code> if
 *  no match is found.
 *     
 *     a = Dvector[ 1, 2, 3, 4, 5, 4, 3, 2 ]
 *     a.index(3)   -> 2
 *     a.index(0)   -> nil
 */ VALUE dvector_index(VALUE ary, VALUE val) {