org-export: Change source code handling API

org-export: Change source code handling API

* contrib/lisp/org-element.el (org-element-example-block-parser,
  org-element-src-block-parser): Add `:number-lines',
  `:preserve-indent, `:retain-labels', `:use-labels'  and
  `:label-fmt' properties.
* contrib/lisp/org-export.el (org-export-resolve-coderef,
  org-export-get-loc): Apply changes to src-block and example-block
  elements' properties.
(org-export-unravel-code, org-export-format-code,
org-export-format-code-default): New functions.
(org-export-handle-code): Removed function.
* EXPERIMENTAL/org-e-latex.el (org-e-latex-example-block): Use new
  function.
(org-e-latex-src-block): Use new API.  Better handling of numbered
lines with special packages.
* EXPERIMENTAL/org-e-ascii.el (org-e-ascii-example-block,
  org-e-ascii-src-block): Use new functions.
* testing/lisp/test-org-element.el: Add tests.
* testing/lisp/test-org-export.el: Add tests.