make src block location available to execution backends
* lisp/ob-core.el (org-babel-where-is-src-block-head): return
point-marker instead of point.
(org-babel-current-exec-src-block-head): new global variable which is
bound during `org-babel-execute-src-block-maybe'
(org-babel-get-src-block-info): return a list of 7 elements, last
being the header location
Evaluation backends might need to use the exact location of the block in
original org file. For example when inserting the correct source
references for visual debugging. With this patch the information on
current block is available in `org-babel-current-exec-src-block-head'.