/*
 *  call-seq:
 *     dvector.where_eq(number)         ->  int or nil
 *     dvector.where_first_eq(number)   ->  int or nil
 *  
 *  Returns the index of the first entry in _dvector_ with value equal to _number_, <code>nil</code> if
 *  _dvector_ has no such entry.
 *     
 *     a = Dvector[ 1, 2, -3, 4, -5, 4, 3, -5, 2 ]
 *     a.where_eq(4)        -> 3
 *     a.where_eq(6)        -> nil
 *     Dvector[].where_eq(4)   -> nil
 */ 
VALUE dvector_where_first_eq(VALUE ary, VALUE item) {