std: Add Vec to the prelude

This is an incredibly common type, and it is expected to be used in many many
places. This type should be in the prelude.
rustc: Default the ~[] lint to allow for now

Most of the standard distribution is still using ~[] instead of Vec, so this
lint is essentially useless currently. When the standard distribution has been
ported to not use ~[], then we can turn the lint back on.
Removing imports of std::vec_ng::Vec

It's now in the prelude.