Requirements: Runtime testing

It would be great to be able to define runtime tests that must be passed as part of the requirements checker. This would involve a severe change in the way the requirements checker works, but my plan is this:

1. Implement the requirements checker as a GHC source plugin rather than a separate CodeWorld build step.
2. For static rules (all of those currently implemented), the result is computed at compile time and hard-coded into the generated code. For dynamic rules (like runtime tests), code is generated to check the result at runtime.
3. Code for reporting (and evaluation, in the dynamic case) of rules is injected at the beginning of `main`, to run as the first action of the compiled code before anything else.