* lisp/arc-mode.el: Use cl-structs rather than vectors

* lisp/arc-mode.el: Use cl-structs rather than vectors

(archive--file-desc, archive--file-summary, archive-arc--file-desc)
(archive-lzh--file-desc, archive-zip--file-desc)
(archive-zoo--file-desc, archive-rar--file-desc)
(archive-7z--file-desc, archive-ar--file-desc): New structs.

(archive-get-descr, archive-mode, archive-summarize-files)
(archive-maybe-copy, archive-extract, archive-*-write-file-member)
(archive-expunge, archive-arc-summarize, archive-arc-rename-entry)
(archive-lzh-summarize, archive-lzh-rename-entry, archive-lzh-ogm)
(archive-zip-summarize, archive-zip-write-file-member)
(archive-zip-chmod-entry, archive-zoo-summarize)
(archive-rar-summarize, archive-7z-summarize, archive-ar-summarize)
(archive-ar-write-file-member): Use struct constructors and accessors
instead of `vector` and `aref`.

(archive-calc-mode): Remove `error` arg which was always non-nil;
adjust all callers.
Rewrite using `string-to-number` and `file-modes-symbolic-to-number`.