Precedence of `box` is possibly wrong

Now that `box` is a keyword, and not the `~` operator, I think it's precedence is wrong. It maintains the precedence of a symbolic unary operator but I think _keyword_ unary operators should have lower precedence.

In practical terms, this means that I think:

```
box 3 + 4
```

should parse as

```
box (3+4)
```

and not

```
(box 3) + 4
```

Nominating.
