Type cast to function pointer doesn't work

See a comment in `Cesium.IntegrationTests/type_casts.c` (introduced in #377):

```

// function pointer types are not supported yet?
// int j = (int) (void (*)(int a)) a;
// int k = (int) (void (*)(int a)) (a + b);
// if (j + k != 5) return -5;
```

We should investigate this problem.

Start with the commented examples in Cesium.IntegrationTests/type_casts.c and review the context from #377. Reproduce the function-pointer casts, determine whether the compiler rejects or mishandles them, and consider the work complete when the behavior is understood and the examples have appropriate passing coverage.

Written by the indexing model from the issue text.