Allow empty struct or provide a way to typealias externals

[derolf]

doesn't work:

@Cpp(EnableIf)
struct FollyIOBuf {
    external {
        cpp include "foo.h"
        cpp name "std::unique_ptr<Foo>"
    }
}

workaround:

@Cpp(EnableIf)
struct FollyIOBuf {
    external {
        cpp include "foo.h"
        cpp name "std::unique_ptr<Foo>"
    }

  dummy: String;
}

I think a better solution would be to allow "external typealiases".

[Hsilgos]

There is a reason why empty struct is not allowed for external types. External type is meant to be serialisable to transfer between languages. struct FollyIOBuf in your example is internal type which keeps some data to serialise/deserialise and then convert to std::unique_ptr<Foo>