Major rewrite. The code is reordered, cleaner and faster.

Major rewrite.  The code is reordered, cleaner and faster.
Introduced new options to automatically cleanup the recent list,
and to handle filename transformation (for example to use true
filenames).
(recentf-version): New constant.
(recentf-save-file-header): Moved.
(recentf-data-cache): New variable.
(recentf-update-menu-p, recentf-initialized-p): Removed.
(recentf-menu-customization-changed): Moved.  Doc fix.
(recentf-max-saved-items): Doc fix.
(recentf-save-file): Doc fix.  No more expand filename here.
(recentf-exclude, recentf-menu-action)
(recentf-menu-filter): Doc fix.
(recentf-menu-append-commands-flag): Renamed from...
(recentf-menu-append-commands-p): Made obsolete.
(recentf-keep-non-readable-files-flag): Renamed from...
(recentf-keep-non-readable-files-p): Made obsolete.
(recentf-auto-cleanup, recentf-filename-handler): New options.
(recentf-string-equal, recentf-string-lessp)
(recentf-string-member): New functions.
(recentf-trunc-list): Moved.
(recentf-dump-variable): Moved.  Better code and output format.
(recentf-auto-cleanup-timer): New variable.
(recentf-auto-cleanup): New function.
(recentf-push, recentf-expand-file-name): New functions.
(recentf-add-file): In-lined.  Use above functions.
(recentf-remove-if-non-readable): In-lined.  Expand file name.
(recentf-find-file): Use `recentf-remove-if-non-readable'.
(recentf-directory-compare): Moved.  Use `recentf-string-equal'
and `recentf-string-lessp' to do comparisons.
(recentf-menu-items-for-commands)
(recentf-menu-filter-commands): Moved.
(recentf-elements, recentf-make-menu-element)
(recentf-menu-element-item, recentf-menu-element-value)
(recentf-set-menu-element-item, recentf-set-menu-element-value)
(recentf-sub-menu-element-p, recentf-make-default-menu-element)
(recentf-menu-elements): In-lined. Some doc fix.
(recentf-apply-menu-filter): Better code.
(recentf-make-menu-items): Doc fix.  Use
`recentf-menu-append-commands-flag'.
(recentf-make-menu-item): In-lined.  Better code.
(recentf-clear-data): New function.
(recentf-sort-ascending, recentf-sort-descending)
(recentf-sort-basenames-ascending)
(recentf-sort-basenames-descending)
(recentf-sort-directories-ascending)
(recentf-sort-directories-descending)
(recentf-show-basenames-ascending)
(recentf-show-basenames-descending: In-lined.  Better code.  Some
doc fix.
(recentf-show-basenames)
(recentf-relative-filter): Better code.  Doc fix.
(recentf-arrange-by-rule-subfilter): Doc fix.  Improved :set code.
(recentf-match-rule-p): Use filename instead of file-path.
(recentf-arrange-by-rule, recentf-build-mode-rules)
(recentf-arrange-by-mode, recentf-build-dir-rules)
(recentf-file-name-nondir)
(recentf-filter-changer-alist): Some doc fix and code cleanup.
(recentf-filter-changer-goto-next): Doc fix.  Call
`recentf-clear-data'.
(recentf-filter-changer-get-current)
(recentf-filter-changer-get-next): In-lined.  Doc fix and better
code.
(recentf-filter-changer): Doc fix and better code.
(recentf-cancel-dialog): Doc fix.
(recentf-dialog-mode-map): Initialized in defvar.
(recentf-dialog-mode): Doc fix.
(recentf-track-opened-file): Renamed from...
(recentf-add-file-hook): Removed.
(recentf-track-closed-file): Renamed from...
(recentf-remove-file-hook): Removed.
(recentf-update-menu-hook): Removed.  Replaced by...
(recentf-update-menu): New.  Better catch unnecessary updates.
Display a message on error.
(recentf-used-hooks): New constant.
(recentf-enabled-p): New function.
(recentf-edit-selected-items)
(recentf-open-files-action)
(recentf-open-files-item-shift): Doc fix.
(recentf-edit-list-action)
(recentf-open-files-item): Doc fix.  Code cleanup.
(recentf-edit-list, recentf-open-files)
(recentf-open-more-files): Likewise.  Removed autoload cookie.
(recentf-save-list, recentf-cleanup): Likewise.  Moved.
(recentf-load-list): New command.
(recentf-mode): Better code.  Does nothing if enabling the already
enabled mode.