/*
 *  call-seq:
 *     dvector.length -> int
 *  
 *  Returns the number of elements in _dvector_.
 *     
 *     Dvector[ 0, -1, 2, -3, 4 ].length   -> 5
 *     Dvector[].length                    -> 0
 */ VALUE dvector_length(VALUE ary) {