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!
The returned `Vec` contains what we call an "uninitialized" raw memory chunk, but from LLVM's point of view, it should just be "some data returned by C function called `je_mallocx()`" and should not be different from, say, "some data returned by C function called `getaddrinfo(3)`."  If you think it's legitimate to read each `struct addrinfo` returned by `getaddrinfo(3)` under LLVM, then it should also be legitimate to read `void*` buffer returned by `je_mallocx()`.  Reading such a buffer should surely be called UB, but it will never lead to deletion of your home directory;  it just yields unpredictable byte sequence.

Hit the [ir] button to see how `Vec` and `getaddrinfo` are translated to LLVM IR:

- http://is.gd/4Czd9Y