org-export: Implement "ref" type links
* contrib/lisp/org-element.el (org-element-link-parser): Recognize
"ref" type links as a new type of Org link.
* contrib/lisp/org-export.el (org-export-resolve-ref-link): New
function.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii--describe-links): Do not
describe "ref" type links at the end of each section.
(org-e-ascii-link): Handle "ref" type links.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-link): Handle "ref" type
links.
"ref" type links are Org answer to LaTeXisms like "\ref{some-label}".
Hence, [[ref:some-label]] will be replaced with the sequence number of
the element with name "#+name: some-label" during export.