Should Ty have copy semantics?

[Trolldemorted]

In #18 I have put several Tys behind Arcs because you have been using them behind references at some places, but the enum feels like a copy type, since Ty<Name> is the variant used most often (only Ty<String> appears to be used in some other places).

[Alasdair]

It's not copy because of the recursive enum variants that contain Box<Ty<A>>. Cloning (and therefore copying) makes a deep copy of the recursive structure.