This allows defining structs and unions inside another struct definition.
Example:
```c
struct a {
 u32 i ;
 struct b {
 u32 ii;
 };
 u32 ia;
 union u {
 u32 ui;
 char uc[4];
 };
 //u au; // warning: this will add another member of the union
};
a a @ 0;
```
The syntax is not exactly the same as C, and it's not anonymous, but at
least you don't need to move the definition out.

Example:
```
struct a {
 struct {
 u32 ii;
 };
 union {
 u32 ui;
 char uc[4];
 };
}
```

```c
struct a {
 struct {
 u32 ii;
 } sname;
 union u {
 u32 ui;
 char uc[4];
 } uname;
}
```