Add sugar to simplify mutable function arguments

While it's pretty easy to write a function that makes an argument mutable:

```
fn foo(x: ~int) {
    let x = move x;
    ...
}
```

It'd be nice to save a line here and there to add some sugar to express an argument is directly mutable, with something like:

```
fn foo(mut x: ~int) { ... }
```
