Replace old owned vectors to std::vec_ng::Vec<T> as possible

To introduce DST into Rust, the old owned vector type ~[T] will be obsolete and will be replaced by std::vec_ng::Vec<T>. So I applied this changes to this library as possible as I can.

Some cases are still using ~[T] because the standard libraries aren't fully migrated to the new Vec<T>, but they'll be not exposed to the external interface.