std::function type cast

[wavewave]

Now that we support std::function, it is desirable to support casting function parameters from Haskell FFI types to C++ types.

Note that Haskell-C++ type/class correspondence is not 1-1 or 1-N, N-1. it's M-N.
For example,
For Function (Ptr RawA -> Ptr RawB -> IO (Ptr RawC)), the corresponding C++ type can be
std::function<(C*)(A*,B*)>, std::function<C(A*,B*)>, std::function<C(A&,B&)> and so on.