Make Calva's nrepl client squint-cljs compatible

[PEZ]

There's a new nREPL server in town: for squint-cljs

Calva fails to connect to it, and fixing that I bump into one error after another preventing me from evaluating anything. CIDER can do it, so I think it highlights things we need to fix in our client.

TODO: Document here what errors I get. (Or I might mention those in the PR that is hopefully coming up).