Simplify RelTables Structure

Some performance hit, but remember that std::vector already does block allocation so it's not too bad.

- Remove block-allocation from RelTables
- Use std::shared_ptr for lst_ptrs
- Replace vector-of-struct-of-vector with vector-of-struct.
- Remove rels from RelTables

include/tc/core.hpp

@@ -141,7 +141,7 @@ namespace tc {
 
         [[nodiscard]] int get(int a, int b) const;
 
-        [[nodiscard]] std::vector<Rel> rels() const;
+        [[nodiscard]] std::vector<Rel> get_rels() const;
 
         [[nodiscard]] SubGroup subgroup(const std::vector<int> &gens) const;