Change HashSet copy constructor from implicit to explicit.

[Ivorforce]

• Similar to Change copy constructors of HashMap and AHashMap from implicit to explicit #115093
• Similar to Change copy constructor of LocalVector from implicit to explicit #115309
• Similar to Change List copy constructor from implicit to explicit. #115646
• Supersedes part of Make most container template copy constructors explicit #111372

This change makes it harder to accidentally copy HashSet instances. This decreases the chance of performance problems and regressions.

Programmers will be forced to take a reference, move, otherwise avoid the copy, or explicitly make the copy:

HashSet<int> hashset;

HashSet<int> other = hashset; // implicit copy, now impossible 

HashSet<int> other(hashset); // explicit copy
HashSet<int> other = HashSet<int>(hashset); // explicit copy
HashSet<int> &other = hashset; // reference
HashSet<int> other = std::move(hashset); // move

This PR is mostly performance agnostic; that is, if a copy was happening beforehand, there will be a copy afterwards (just explicit instead of implicit). There are a few minor exceptions that I'm very sure are safe.

[Repiteo]

Thanks!