Potential performance optimizations

[dimstav23]

• split_comma_string: change it to return vector of std::string_view (small effect probably)
• instructions like m_objection = std::bitset<num_purposes>(std::stoull(std::string(token))); : Is there any way to avoid copying? Check their frequency to determine whether it makes sense
• query_rewriter: can we simply change the result in place? We might have an issue with the sizing. Maybe copy only when resizing is needed.