remove old_iter

the `test/run-pass/class-trait-bounded-param.rs` test was xfailed and
written in an ancient dialect of Rust so I've just removed it

this also removes `to_vec` from DList because it's provided by
`std::iter::to_vec`

an Iterator implementation is added for OptVec but some transitional
internal iterator methods are still left
Fix/annotate a variety of xfails
xfail test again for now (issue #7336)
deal with windows
implement Clone for slices