Add improved marshaling support for pointer types

[Znurre]

When marshaling a pointer type, be it as a reference type or an opaque pointer, we need to create boilerplate for checking for null pointer.

Basically, every pointer exposed to FluenC from C, shall expand to this pseudo code:

if (pointer)
{
    return mapToStructTypeAsPerUsual(...);
}

return nothing;

This will force the user of the C API to handle the case where the pointer might be null.