`$next` isn't usable in setting virtual fields

Suppose I have the following Emboss definition:

```
struct Foo:
0 [+1] UInt bar
$next [+1] UInt baz

let version_0_01_size = $next
```

If I wanted to store the current offset in a virtual field such that I can access it in code later, I'm currently unable to do this. I get the following error:

```
error: Keyword `$next` may not be used in this context.
```

I think this would be pretty useful to have so as to avoid repeating values or potentially make mistakes in counting offsets.