`FuncPtr` signature checking

After #356, Cesium now supports `FuncPtr` interop: you can pass a C function pointer to a .NET method that has `FuncPtr` in its signature.

We should check the signatures of these functions (i.e. check if `TDelegate` is a corresponding `Action` or `Func`). And also while we are at it, figure out the signature mapping: in particular, how to emphasize the pointers in the signature.

These signatures of `TDelegate` aren't currently used anywhere in runtime, so they are mostly a compile-time thing.

See `TODO[#490]` in the code to look where to implement.