When extracting a C struct - e.g.

```
struct Point {
   int x;
   int y;
}
```

Jextract emits hardwired offsets:

```
class Point {
   ...
   private static final long x$OFFSET = 0;
   ...
   private static final long y$OFFSET = 4;
}
```

While this is good for startup, it makes it difficult to alter the layout of a struct after the fact - e.g. by adding platform-dependent padding -- which will make all the offsets incorrect.

It would be better/more robust if the generated code looked like this:

```
class Point {
   private static final long x$OFFSET = LAYOUT.byteOffset(groupLayout("x"));
...
   private static final long y$OFFSET = LAYOUT.byteOffset(groupLayout("y"));
...
}
```