That saves us the complex code to display the list of completions.
Unfortunately, slime-complete-symbol-function doesn't work well to
customize it.  Had to introduce a replacment:
slime-completion-at-point-functions.

* slime.el (slime-complete-symbol): Use completion-at-point.
(slime-completion-at-point-functions): New.
(slime--completion-at-point-functions): New helper.
(slime-complete-symbol-function): Obsolete, but have to keep it
around for some time.
(slime-simple-completion-at-point): Renamed from
slime-simple-complete-symbol to avoid confusion.
(slime-filename-completion): Renamed from
slime-maybe-complete-as-filename.
(slime-simple-complete-symbol): Backward compatible replacement.

(slime-completions-buffer-name)
(slime-complete-saved-window-configuration)
(slime-completions-window)
(slime-complete-maybe-save-window-configuration)
(slime-complete-delay-restoration)
(slime-complete-forget-window-configuration)
(slime-complete-restore-window-configuration)
(slime-c...