Support function pointers and use a state monad in the frontend.
Important changes in the core semantics: * Types extended with function types. Since function types are a special kind of pointer types, types now have an additional mutual part called "ptr_type". * Pointers extended with function pointers. Theses are just names that refer to an actual function in the function environment. * Typing environments extended to assign argument and return types to function names. Before we used a separate environment for these, but since the argument and return types are already needed to type function pointers, this environment would appear in pretty much every typing judgment. As a side-effect, the frontend has been rewritten entirely. The important changes are: * Type checking of expressions is more involved: there is a special kind of expression type corresponding to a function designator. * To handle things like block scoped extern function, more state-fullness was needed. To prepare for future extensions, the entire frontend now uses a state monad.