* ses.el (ses-cell-formula-aset): New macro.

* ses.el (ses-cell-formula-aset): New macro.
(ses-cell-references-aset): New macro.
(ses-cell-p): New function.
(ses-rename-cell): Do no longer rely on complex operations like
ses-cell-set-formula or ses-set-cell to change the cell and handle
the undo at the same time, but rather use lower level new macros
`ses-cell-formula-aset' and `ses-cell-references-aset' and handle
the undo directly. Refresh the mode line.