Support struct "inheritance" (extension)

C11 supports struct extension, such that:

```
struct Foo {
byte foo1
}

struct Bar {
byte bar1
}

struct Foo_bar {
struct Foo
struct Bar
}
```
Produces the final struct:
```
struct Foo_bar {
byte foo1
byte bar1
}
```

While not a make or break feature, this allows some convenience in allowing composition without nesting, which is already represented by the memory structure.