Finish the removal of mc::api by moving the last bits to the side

Finish the removal of mc::api by moving the last bits to the side

I would prefer to directly kill the templated structure, but I fail to
write a correct comparison lambda because it's used over a
std::vector<std::unique_ptr<simgrid::mc::VisitedState>>

All my attempts lead to either "no conversion to std::unique_ptr<VisitedState>",
or an error saying that I try to copy a unique_ptr when my lambda
takes unique_ptr as a parameter.

Please someone better at C++ than I am, please kill it.