lint: add lint for use of a `~[T]`.

This is useless at the moment (since pretty much every crate uses
`~[]`), but should help avoid regressions once completely removed from a
crate.
std: render the vec_ng docs.

These are wildly incomplete, but having something there is better than
nothing, e.g. so that people know it exists, and many of the functions
behaviour can be guessed from the name or by checking the source: it's
knowing they exist at all that's the hard part.