Add lifetime to solving such that clone_for_plugins is not leaking

[Andful]

As part of #151, I added a lifetime to Solving to prevent clone_for_plugins from leaking.

This requires include_branch_rule, include_heur and include_pricer to not only accept "callbacks" of 'static lifetimes, but also "callbacks" of shorter lifetime.

This means that include_branch_rule, include_heur and include_pricer cannot consume self :/, as the callback should live as much as model is live. This goes a bit against the builder pattern that this library uses.

Would like some comments on the code. I still have to think hard if this code does not have memory errors.