Use generic Marshal.GetDelegateForFunctionPointer for explicitly cast closed source delegates

Replace explicitly cast, resolved GetDelegateForFunctionPointer(pointer, typeof(T)) calls with the generic overload for the same closed source delegate on known .NET Framework 4.5.1+, .NET Standard 1.2+ or .NET Core 1.0+ targets. Preserve the cast, pointer evaluation, ownership and method binding. Admit primitive, pointer-sized and raw-pointer signatures with known marshalling annotations; skip generic or unsupported signatures, ambiguous projects, old language syntax, comments and conditional snapshots.