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!
I'm aware that Rust is not currently communicating that the memory is uninitialized *in a build without --disable-jemalloc*. LLVM special cases certain libc symbols like `malloc`, and it does infer this information from the symbol name along with doing stuff like dead store elimination of dynamic allocations. The fact that it isn't communicated to LLVM for other allocator symbols is a performance bug. It is *not* a guarantee that you're allowed to leverage. Note that the reference manual currently states that reading uninitialized memory is UB.