org-element: Optimize affiliated keywords parsing

org-element: Optimize affiliated keywords parsing

* lisp/org-element.el (org-element-center-block-parser,
  org-element-drawer-parser, org-element-dynamic-block-parser,
  org-element-footnote-definition-parser,
  org-element-plain-list-parser, org-element-quote-block-parser,
  org-element-special-block-parser, org-element-babel-call-parser,
  org-element-comment-parser, org-element-comment-block-parser,
  org-element-example-block-parser, org-element-export-block-parser,
  org-element-fixed-width-parser, org-element-horizontal-rule-parser,
  org-element-keyword-parser, org-element-latex-environment-parser,
  org-element-paragraph-parser, org-element-property-drawer-parser,
  org-element-src-block-parser, org-element-table-parser,
  org-element-verse-block-parser): Change signature. Now use an
  additional argument: affiliated.
(org-element--current-element): Skip affiliated keywords and pass them
  as an argument to parsers.  It prevents to walk through these
  keywords twice: the first time to get to the first line of the
  element, and a second time to collect the affiliated keywords.
* contrib/lisp/org-export.el (org-export-get-caption): Update function
  since multiple affiliated keywords are now reversed.
* testing/lisp/test-org-element.el: Update tests.