Refactor ZiprUntypedOption_t, add observee field and

check for matching types on AddObserver

1. Refactor ZiprUntypedOption_t -> ZiprOption_t
2. Add observee field: this will let an observer
track its observee. When () or == (etc) are called,
they should pass control to their observee.
3. Add type checking for AddObserver.