C++: Generate "arg" version of structs

[koplyarov]

Problem

Consider the following joint struct:

// Something.idl
struct MyStruct
{
    i32 i;
    IObject obj;
    string str;
}

The C++ code, generated for this struct:

// Something_adapters.hpp
struct MyStruct
{
    int32_t i;
    joint::IObject_Ptr obj;
    joint::String str;

    // constructors, operators, ToString method, etc.
};

The generated C++ struct holds strings and intrusive object pointers by value. Both string copying and atomic operations are unnecessary when the struct is passed as a parameter into a C++ method:

class MyComponent
{
    using JointInterfaces = joint::TypeList<IMyInterface>;

    /**
     * MyStruct instance is constructed somewhere in the accessor code for IMyInterface, 
     * copying the str data and incrementing the atomic reference counter of obj.
     * Both operations are unnecessary for this particular method.
     */
    void Func(const MyStruct& s)
    { std::cout << "s.str: " << s.str << std::endl; }
};

What to do

1. Also generate <StructName>_Arg C++ structure that does not own any expensive-to-copy data somewhere around here

// Something_adapters.hpp
struct MyStruct_Arg
{
    int32_t i;
    joint::IObject_Ref obj;
    joint::StringRef str;

    // constructors, operators, ToString method, etc.
};

2. Construct arg struct version for passing to component methods:

• add ref_type parameter to CppType constructor here
• remove recursive parameter and check here

JFYI, here's the unmarshaling of the parameters in C++ adapters template