Implement missing ns-related functionality

While looking at the `clojure-project-dir` functionality, I've noticed that we haven't yet ported any of the `ns` manipulation commands from `clojure-mode` (see https://github.com/clojure-emacs/clojure-mode/blob/master/clojure-mode.el#L2044)

Seems we're also missing something like `clojure-find-def`, although I'm not sure this was used much in practice.

I also remembered another reason why `clojure-project-dir` was in `clojure-mode` and not CIDER - it was needed by `clojure-expected-ns`.