Marshal/unmarshal Ice::Value by value in C++

[bernardnormier]

As of Ice 3.7, it's not possible to marshal or unmarshal an Ice::Value by value in C++, it can only be done for heap allocated value held by a shared_ptr.

Ice doesn't need such support for its C++ mapping but with DataStorm, users might very well want to use Slice classes as topic values and not have to go through shared_ptr<T>.