Show locals table at each step in debugger

[AdamClements]

Rather than simply showing the last form return value, use the contents of &env to display a table of all the local bindings at a given point, as discussed with @Malabarba in gitter.

(defmacro locals [] (zipmap (map #(list 'quote %) (keys &env)) (keys &env)))

This will likely include some autogenerated locals (for destructuring, macros, etc.). These aren't likely to be too distracting, but a simple heuristic of whether a name consists of an underscore #[_] should filter out all of these (maybe have a toggle?)

(defn remove-autogenerated-locals
  "Remove any locals generated by macros (heuristic - named something
  with underscores in it). When we're doing something other than
  println with the data, these could be left in but it's a bit noisy
  at the moment"
  [coll]
  (into {} (remove (fn [[k v]] (re-find  #"[_]" (name k))) coll)))

[Malabarba]

If anyone wants to try this the clojure side is trivial. The breakpoint macro already holds the list of local variables in *locals*. So it's just a matter of sending the contents of that variable (possibly sanitized as above) when we send the message to Emacs.

The Emacs side is then just a matter of displaying that information somewhere.

[Malabarba]

Related: #1073 (comment)