`requires` on a field cannot reference other fields

Suppose I had the following Emboss definition. Such a definition creates a requirement bound on the value of `bar` while also exposing `max_value` virtual field programmatically via `Foo::max_value()`. It allows developers to keep the minimum or maximum bounds defined in one place and referred to elsewhere, including their code.

```
struct Foo:
let max_value = 128
0 [+1] UInt bar
[requires: 0 <= this <= max_value]
```

However, this is currently not possible. According to the [language reference](https://github.com/google/emboss/blob/master/doc/language-reference.md#requires), "for `[requires]` on a field, other fields may not be referenced."

There is a workaround:

```
struct Foo:
[requires: 0 <= bar <= max_value]
let max_value = 128

0 [+1] UInt bar
```

While this accomplishes the same goal, I would say it's not as good as being able to refer to other fields from a `[requires]` on a field itself. If I had multiple of these requirement blocks, it could get pretty unwieldy to have them all at the top instead of closer to where they semantically matter.