org-element: Implement caching for dynamic parser

org-element: Implement caching for dynamic parser

* lisp/org-element.el (org-element-use-cache, org-element--cache,
org-element--cache-sync-idle-time,
org-element--cache-merge-changes-threshold, org-element--cache-status,
org-element--cache-opening-line, org-element--cache-closing-line): New
variables.
(org-element-cache-reset, org-element--cache-pending-changes-p,
org-element--cache-push-change, org-element--cache-cancel-changes,
org-element--cache-get-key, org-element-cache-get,
org-element-cache-put, org-element--shift-positions,
org-element--cache-before-change, org-element--cache-record-change,
org-element--cache-sync): New functions.
(org-element-at-point, org-element-context): Use cache when possible.
* lisp/org.el (org-mode, org-set-modules): Reset cache.
* lisp/org-footnote.el (org-footnote-section): Reset cache.
* testing/lisp/test-org-element.el: Update tests.

This patch gives a boost to `org-element-at-point' and, to a lesser
extent, to `org-element-context'.