rewrite checker in C

We now generate the checker itself as C, rather than C++. My current intuition
is that we can beat the speed of the C++ checker by moving to C. By making our
operations more transparent to the compiler we should be able to (a) move type
checking to Rumur and avoid some coercion hell in the checker as well as (b)
essentially force devirtualising all function calls.

Note that this commit drops a significant amount of functionality:

  * TBB support
  * Useful state hash (to be reintroduced later)
  * Multithreaded support (to be reintroduced later)
  * Support for a fixed-size seen set
  * Support for a fixed-size queue