Extern SEEC_PP_APPLY to support variadic lists of up to 20 arguments.
Add input, value, and linking information for C Decl nodes.

Add an X-Macro system for implementing this, based on the Stmt X-Macro system.
The Decl system also allows specifying information for abstract classes, and
ensures that a node will have the information added for the node's class, as
well as all of its base classes.