cider-jump-to-def should use a 'var-info' op if present.

[gtrak]

Much like nrepl-complete [#460], we need an op and nrepl middleware to remove the reliance on 'eval' for cljs support.

Current code builds a huge string:
https://github.com/clojure-emacs/cider/blob/master/cider-interaction.el#L492

I am happy to work on such a middleware. If needed, I can try to implement the elisp op call and eval fallback.

var-info will take an ns and var name and return file+line, doc, arglists and whatever else is in common between clj and cljs.

[bbatsov]

Indeed. Such a middleware should also be part of cider-nrepl. My long term plan is to remove all non-trivial & non-portable inlined code and to replace it with nREPL middleware.