Strategy pattern with templates

[Kalashnikovni]

Currently, we've implemented the strategy pattern with pointers, which allows for runtime configuration, but it may not be as performant as a template-based strategy. This last option is feasible, since we execute all causalization with a fixed implementation.

[Kalashnikovni]

In the end, I've chosen to use a std::variant inside Set for the following reasons:

• It is as fast as the Strategy pattern. I've run tests with the UnorderedSet implementation in both versions, and the times where similar or better than its predecessor.
• Adding a new implementation is cheap, and only implies some additions in specific modules. Set member functions don't require modifications.
• It keeps a clean interface for users, eliminating the need of writing templated functions.