* lisp/icomplete.el (icomplete-in-buffer): New var.

* lisp/icomplete.el (icomplete-in-buffer): New var.
(icomplete-pre-command-hook, icomplete-post-command-hook): Remove those
vars and replace them with functions.
(icomplete-minibuffer-setup): Adjust accordingly.
(icomplete--completion-table, icomplete--completion-predicate)
(icomplete--field-string, icomplete--field-beg, icomplete--field-end):
New functions.
(icomplete-forward-completions, icomplete-backward-completions)
(icomplete-simple-completing-p, icomplete-exhibit)
(icomplete-completions): Use them.
(icomplete--in-region-buffer): New var.
(icomplete--in-region-setup): New function.
(icomplete-mode): Use it.