Dependent routes/data structures?

Is there a way to indicate dependencies between routes? For example very often, visiting `/login` is required before `/logout`.

There could also be dependencies between data, let's say you have:
```
alias UUID = String(pattern="[0-9a-fA-F]{8}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{4}\-[0-9a-fA-F]{12}")

struct RegisterReq
"Describes a user registration request, this is often required to do anything else."

username String
"Given username for that user."

struct RegisterResp
"Describes a registration response, indicates a user has registered and is now identifiable"

uuid UUID
"A universal identifier"

struct LogoutReq
"Triggers the user to logout, requires a UUID to have been received"

@Depends(RegisterResp)
uuid UUID

route register_person(RegisterReq, RegisterResp, Void)
route logout_person(LogoutReq, Void, Void)
```

I think dependency semantics are very useful when testing routes, so if there's no semanics in place right now, I'd like to propose a `@Depends` annotation for dependant datatypes, from which we can infer the natural order of requests, used as seen above.