org-element: Change data structure for cache
* lisp/org-element.el (org-element-at-point, org-element-cache-get,
org-element-cache-put, org-element--cache-sync): Complete rewrite to
use new data structure.
(org-element-context, org-element-cache-reset): Slight change in order
to use new tools to access cached data.
(org-element-cache-merge-changes-threshold): Renamed from
`org-element--cache-merge-changes-threshold'.
(org-element-cache-sync-idle-time): Renamed from
`org-element--cache-sync-idle-time'.
(org-element--cache-objects): New variable.
Now elements are stored in AVL tree and objects in a hash table. Also
moved functions relative to cache into a specific section of the file.