profiling support

These changes add three ops to support profiling via the thunknyc/profiling library: `toggle-profile`, `print-summary`, and `clear-profile`.

* `toggle-profile` works analogously to `toggle-trace`.
* `profile-summary` returns a tabular summary of collected profiling stats via `:err`.
* `clear-profile` clears any collected data from the profiling session.

I've also written the Emacs-side code that drives these ops; they'll be packaged up in a pull request to cider-nrepl/cider.