ObjFW  Changes To API Review for 1.0

* ✅ `ObjFWTLS/ObjFWTLS.h`

Some things still need to be checked on a global level:

* ❌ Classes that already reserve ivars need to be considered to be changed to restricted subclassing
* ❌ All classes either restrict subclassing or reserve ivars
* ❌ All exception throws need to be audited (they're part of the API, too!)
* ❌ OFSerialization is up for consideration to be removed