org-element: Speed optimizations
* contrib/lisp/org-element.el (org-element--element-block-re): New
variable.
(org-element-current-element): New function.
(org-element-parse-elements): Make use of the new specialized function
instead of `org-element-at-point'. Also narrow buffer to current
container (greater element or recursive object) being parsed to
improve search speed.
(org-element-comment-parser): Speed-up parsing for comments at column 0.
(org-element-guess-type): Make comment regexp less restrictive, due to
comment optimizations.