Vec: add uninitialized(uint) and resize(&mut self, uint, T).

Add two more convenience methods to `Vec`.

Please review my English in the rustdoc comments!
As thestinger notes, this can actually be written entirely safely using grow and truncate. As such I'm inclined to just let people do this manually (otherwise this opens up a whole can of worms for things like `resize_with`). It's easy enough to add it via an extension trait if you really want it.

CC @aturon