Delete overlays when clearing REPL buffer

[rfkm]

I've noticed that zombie overlays for ANSI coloring in the REPL buffer seem to impact its performance.

For example, evaluate the following code:

(dotimes [_ 100]
  (println "\u001b[31mfoo\u001b[0m"))

Then, type C-u C-c C-o to clear the repl buffer.

Finally, you can confirm empty overlays are still there by evaluating the following elisp code in the REPL buffer:

(length (overlays-in (point-min) (point-max))) ; => 100

In my case, the number of zombie overlays reached more than 20000 after long working and REPL buffer's response was very slow.
In this PR, I tweaked cider-repl-clear-output and cider-repl-clear-buffer to remove overlays.

[bbatsov]

👍 Nice catch!

[cichli]

Very nice catch indeed, thanks for your recent contributions @rfkm!

[rfkm]

Glad to hear that! CIDER is one of the most important OSS for me, so I'm very happy to help.
Thanks for the great project! :-)