editor/format: format by diff & major refactor

This redesigns how format-all applies changes. Before, it would erase
the buffer and insert the formatted result. This would throw markers out
of whack, and usually meant the cursor would end up somewhere else (e.g.
the beginning of the file).

Now, changes are made to the buffer line by line, via RCS patch.

Other changes:

+ +format-region function was removed (replaced by +format-buffer and
  +format/region).
+ set-formatter! was moved to autoload/settings.el
+ New +format/buffer command to replace format-all-buffer

modules/editor/format/autoload/evil.el

@@ -3,6 +3,6 @@
 
 ;;;###autoload (autoload '+format:region "editor/format/autoload/evil" nil t)
 (evil-define-operator +format:region (beg end type)
-  "Evil ex interface to `+format-region'."
+  "Evil ex interface to `+format/region'."
   (interactive "<R>")
-  (+format-region beg end))
+  (+format/region beg end))