Add tests for smart pointers passed by value

[Geod24]

Currently our smart pointers bindings (std::shared_ptr & co) do not have a copy constructor defined. In order to avoid issues, we pass them by reference.

However, upstream stellar changed their SCP implementation and some smart pointers are passed by value. Hence, we need to add support for that in order for our bindings to be usable.

TODO:

• Add the required copy constructors to our smart pointers, for all supported platforms;
• Add a test here to ensure that pass-by-value works correctly, as well as returning. So C++ to D by value, D to C++ by value, return by value from C++ to D, return by value from D to C++.

Be wary of bugs.

[Geod24]

We're affected by https://issues.dlang.org/show_bug.cgi?id=20235
I'll raise the point in the D foundation meeting on Friday.

[Geod24]

So the tests hasn't been checked in because they fail - @ferencdg found a bug that is a blocker for us. In that regardm I consider this issue closed, since it achieved its aim to verify if things worked or not.