Converge blows up with low-value error message.

When `converge` is invoked with a copy of the CRDT which is remote copy as the first arg, and the local copy as the second arg, it may provoke a null pointer exception. This is not an intuitive problem to diagnose. We can introduce a concept of `local-copy?` of a structure and when the first arg to `converge` is not `local-copy?`, fail.