refactor smart_c to use contract by design and expected

[elfenpiff]

Brief feature description

The SmartC class should get the two methods and_then and or_else so that it can be used like the optional or `expected.

Detailed information

• remove getErrorString from SmartC
• make all methods in SmartC private (even ctor since makeSmartC is only using this)
• consider removing the SmartC class and use only free functions
• It should be possible to use SmartC like

cxx::makeSmartC(sigaction, cxx::ReturnMode::PRE_DEFINED_SUCCESS_CODE, {0}, {}, SIGINT, &act, nullptr)
     .and_then([&](auto sigActionReturnValue){ /* do stuff */ })
     .or_else([&](auto sigActionErrorReturn){ /* report error */ });

• Find a nicer solution for this use case where ignoring SomeErrorCode:

        if (smartcCall.hasErrors())
        {
            // sad path
        }
        else if (connectCall.getErrNum() == SomeErrorCode)
        {
           // something inbetween happy and sad
        }
        else
        {
            // happy path
        }