This is an owned sendable linked list which allows insertion and
deletion at both ends, with fast traversal through iteration, and fast
append/prepend.
It is indended to replace the previous managed DList with exposed list
nodes. It does not match it feature by feature, but DList could grow
more methods if needed.
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
