`Base.similar` method for acsets

There are some places in the code (for instance in the nauty integration, but I bet not just there) where we copy or even `deepcopy` a whole acset just to get an acset of the same shape that we'll then overwrite. It might be nice to write a method of `Base.similar` that copies an acset's shape (i.e. attrtypes and cardinalities of homs) but leaves everything uninitialized for this kind of situation. I don't know enough about columns to know whether this would be trivial or annoying.