Port: a minimalist prepl client for Emacs

For ages I’ve had “add prepl support to CIDER” sitting somewhere in the back of my head. CIDER is built firmly around nREPL, but prepl ships with Clojure itself, and the appeal of dropping the external REPL server requirement is obvious. Recently, as part of a broader internals cleanup “mini-project” in CIDER, I finally sat down and put a prototype together: cider#3899.