Ticket 333: Require eof-objects to be disjoint from basic Scheme types

It's already a requirement that an eof-object cannot have an external representation, which means it cannot be any of the basic types in Section 3.2 except procedure or port. This is very improbable, and in fact none of my 40 test schemes returns either a procedure or a port.

Doing this would allow eof-object? to be added to the list of disjoint type predicates in Section 3.2.