ob-exp: Consolidate export of code blocks

ob-exp: Consolidate export of code blocks

* lisp/ob-exp.el (org-babel-exp--at-source): New macro.
(org-babel-exp-in-export-file): Remove macro.
(org-babel-exp-src-block): Remove unused argument.  Use new macro.
(org-babel-exp-process-buffer): Apply signature change above.  Put
temporary properties on top of Babel code so as to find original
location without relying on heuristics.
(org-babel-exp-results): Use new macro.

Export process has to know context of Babel code being evaluated (e.g.,
so as to retrieve parameters from node properties).  However, evaluating
previous code can drastically change the buffer.  The library used some
heuristics to find the original location.  Those are not necessary
anymore.