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!
`Int` is the most general trait which includes `u8` (expected to be the most frequent use case for this ctor)  but is known not to imply `Drop` (we should forbid allocating a `Vec` of RAII objects such as `Vec<Vec>` with this ctor.)  [According to the document](http://doc.rust-lang.org/std/num/trait.Int.html), `Int` is

> A built-in signed or unsigned integer.

What problem do you expect with `Int`?  Do you think it is too specific?