/*
 *  call-seq:
 *     dvector.each_index {|index| block }  ->  dvector
 *  
 *  Same as <code>Dvector#each</code>, but passes the index of the element
 *  instead of the element itself.
 *     
 *     a = Dvector[ 1, 0, -1 ]
 *     a.each_index {|x| print x, " -- " }
 *     
 *  produces:
 *     
 *     0 -- 1 -- 2 --
 */ VALUE dvector_each_index(VALUE ary) {