spee-up table refresh

[Thierry Banel <tbanelwebmin@free.fr>]

Re-computing a large table is slow. Hereafter is a test case for a 1000
row table. My computer refreshes it in 44 seconds.

Here is a fast-and-dirty-not-to-be-commited patch which speed-up the
refresh to less than 1 second. I just removed the last 3 lines of
(org-at-table-p).

I don't understand those 3 lines, but everything still works fine
without them (the 688 tests work as expected). Those lines indirectly
call (org-element--cache-put) a quadratic number of times. For a 1000
rows table this is 501500 times (about 1000x1000/2 times).

#+BEGIN_SRC elisp :results none
(defun org-at-table-p (&optional table-type)
  "Non-nil if the cursor is inside an Org table.
If TABLE-TYPE is non-nil, also check for table.el-type tables.
If `org-enable-table-editor' is nil, return nil unconditionally."
  (and
   org-enable-table-editor
   (save-excursion
     (beginning-of-line)
     (looking-at-p (if table-type "[ \t]*[|+]" "[ \t]*|")))
))
;   (or (not (derived-mode-p 'org-mode))
;       (let ((e (org-element-lineage (org-element-at-point) '(table) t)))
;     (and e (or table-type (eq (org-element-property :type e) 'org)))))))
#+END_SRC

Here is a test case.

1- First create a 1000 row table:

#+BEGIN_SRC elisp :results none
  (goto-char (point-max))
  (let ((i 1000))
    (while (> i 0)
      (insert (format "| %4s | |\n" i))
      (setq i (1- i))))
  (insert "#+TBLFM: $2=$1*10\n")
#+END_SRC

2- Then, with point in the table, type C-u C-c *

Regards,
Thierry