Parse multiple consts from a single keyword

This feels like it should work:

```
const bar: int = 1,  // This should mirror the `let` syntax
      foo: int = 4;

fn main() {
    let baz: int = 0,
        qux: int = 1;
}
```

But instead you get:

```
const.rs:1:18: 1:19 error: expected `;` but found `,`
const.rs:1 const bar: int = 1,
                             ^
```
