Add basic functors for the category of sets

[ScriptRaccoon]

This PR adds these functors to the database and decides all of their properties:

• the squaring functor $Set \to Set$, $X \mapsto X^2$
• the doubling functor $Set \to Set$, $X \mapsto 2X$
• the diagonal functor $Set \to Set \times Set$, $X \mapsto (X,X)$
• the binary product functor $Set \times Set \to Set$, $(X,Y) \mapsto X \times Y$
• the binary coproduct functor $Set \times Set \to Set$, $(X,Y) \mapsto X + Y$
• the sequences functor $Set \to Set$, $X \mapsto X^{\omega}$
• the countable copower functor $Set \to Set$, $X \mapsto \omega \times X$