`FuncPtr` signature checking

[ForNeVeR]

After #356, Cesium now supports FuncPtr interop: you can pass a C function pointer to a .NET method that has FuncPtr<TDelegate> 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.