IDL43 — optional should be a keyword, not an annotaton

The annotation optional impacts the language mapping, for example in C it results in a pointer, with C++11 it could be a IDL::optional<>. The usage of an annotation is very weak in terms of semantics, it is much better to use a new optional keyword as optional heavily impacts the type presented to the programmer. Adding optional will break existing user code, it is not only something that is checked by a concrete middleware.