* lisp/textmodes/rst.el: Major refactoring, minor changes, minor fixes

* lisp/textmodes/rst.el: Major refactoring, minor changes, minor fixes

(rst-Ado, rst-Hdr, rst-Ttl, rst-Stn): Introduce classes
representing reStructuredText section header concepts.
(rst-mode-map, rst-new-preferred-hdr)
(rst-update-section, rst-classify-adornment)
(rst-ttl-at-point, rst-all-ttls-cache)
(rst-hdr-hierarchy-cache, rst-reset-section-caches)
(rst-all-ttls, rst-infer-hdr-hierarchy, rst-hdr-hierarchy)
(rst-all-ttls-with-level, rst-get-previous-hdr)
(rst-adornment-complete-p, rst-next-hdr, rst-adjust)
(rst-adjust-section, rst-promote-region)
(rst-display-hdr-hierarchy, rst-straighten-sections)
(rst-all-stn, rst-remaining-stn, rst-toc-insert)
(rst-toc-insert-node, rst-toc-node, rst-toc)
(rst-forward-section, rst-adornment-level)
(rst-font-lock-handle-adornment-pre-match-form)
(rst-imenu-convert-cell, rst-imenu-create-index): Refactor
using classes.
(rst-compare-adornments, rst-get-adornment-match): Remove
functions now in classes.
(rst-re-alist-def, rst-toc-mode)
(rst-font-lock-extend-region-extend): Minor improvements.
(rst-mode, rst-compile): Use `setq-local'.
(rst-cvs-header, rst-svn-rev, rst-svn-timestamp)
(rst-official-version, rst-official-cvs-rev)
(rst-package-emacs-version-alist): Maintain version tags.