Implement numbered cross-references

Implement numbered cross-references

* lisp/org.el (org-link-search): Search for #+name affiliated keywords
  and invisible targets.
* contrib/lisp/org-element.el (org-element-link-parser): Remove "ref"
  links relative part.
(org-element-target-parser): Move property name from `:raw-value' to
`:value'.
(org-element-recursive-objects): Remove targets from tables.  Cells
are not parsed unless explicitely asked by back-end developer, too
late.  A target wouldn't be noticed in time.  One solution could be to
parse every table, but that's time consumming.
(org-element-object-restrictions): Target are not recursive anymore.
* contrib/lisp/org-export.el (org-export-resolve-fuzzy-link): Find
  elements with a matching "#+name: path" affiliated keyword.
(org-export-get-ordinal): Make special cases for headlines, items,
footnotes definitions and references.
(org-export-resolve-ref-link): Removed function.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-link): Handle
  cross-reference numbers.
(org-e-latex-target): Targets have no contents.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii--describe-links): Ignore
  fuzzy links in link description at the end of the section.
(org-e-ascii-link): Handle cross-reference numbers.
* testing/contrib/lisp/test-org-export.el: Add tests.
* testing/lisp/test-org.el: Add tests.