org-element: Split tables into table-row elements and table-cell objects

org-element: Split tables into table-row elements and table-cell objects

* contrib/lisp/org-element.el (org-element-table-parser): Split tables
  into table-row elements and table-cell objects.
(org-element-table-interpreter): Adapt interpreter to new code.
(org-element-table-row-parser, org-element-table-row-interpreter,
org-element-table-cell-parser, org-element-table-cell-interpreter,
org-element-table-cell-successor, org-element-table-row-successor,
org-element-restriction): New functions.
(org-element-headline-parser,
  org-element-inlinetask-parser, org-element-item-parser,
  org-element-verse-block-parser,
  org-element-footnote-reference-parser,
  org-element-collect-affiliated-keywords, org-element-parse-objects):
  Use new function
(org-element-all-objects): Add new objects.
(org-element-target-parser): Small change to docstring.
(org-element-object-restrictions): Merge `org-element-string-restrictions'
into it.
(org-element-string-restrictions): Remove variable.
(org-element-parse-elements): Parse objects in non-recursive elements
with contents.
(org-element-normalize-string): Small refactoring.
(org-element-at-point): Handle table navigation.
* testing/lisp/test-org-element.el: Add tests.