* rst.el: Integrate support for `imenu' and `which-function'.

* rst.el: Integrate support for `imenu' and `which-function'.
Fixes feature request bug#11711.

(rst-mode): Create `imenu-create-index-function'.
(rst-get-stripped-line): Delete after refactoring.
(rst-section-tree, rst-section-tree-rec)
(rst-section-tree-point): Refactor and document properly.
(rst-imenu-find-adornments-for-position)
(rst-imenu-convert-cell, rst-imenu-create-index): New
function.