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 tests again for now (issues #7336, #7340)
deal with windows
implement Clone for slices
auto merge of #7334 : thestinger/rust/old_iter, r=Aatch

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