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.