Struct-level `[requires]` aren't enforced on field writes

Suppose I have the following Emboss definition:

```
struct Foo:
[requires: bar == 0]
0 [+1] UInt bar
```

If I were to write a non-zero value into `bar`, `emboss_prelude.h:185 EMBOSS_CHECK(result);` wouldn't execute and the write would succeed. It wouldn't be until I called `bool Ok()` that I would find out that something was written incorrectly. At that point, I just get a boolean `true` or `false` and not a directed, clear result of what went wrong.

This isn't the case if I place the `[requires]` block on the field itself:

```
struct Foo:
0 [+1] UInt bar
[requires: this == 0]
```

This second Emboss definition would catch the non-zero write via `emboss_prelude.h:185 EMBOSS_CHECK(result);`. Regardless of where the `[requires]` block is placed, we should catch incorrect writes when they happen.