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!
There has been some informal discussion of potentially killing assert_eq in favour of richer pattern matching in the macro. So like assert!( foo == bar) compiles to what assert_eq is now, with knowledge of LHS and RHS. And assert!( foo < bar) compiles to something similar but for inequality. 

I don't know if there's an official bug for this.