Support struct "inheritance" (extension)

[agg23]

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.