* ses.el: Use hash map for getting named cells coordinates.

* ses.el: Use hash map for getting named cells coordinates.
 symbol to coordinate mapping is made by symbol property
 `ses-cell'. This means that the same mapping is done for all SES
 sheets. That is good enough for cells with standard A1 names, but
 not for named cell. So a hash map is added for those
 latter.
 (defconst ses-localvars): added local variable ses--named-cell-hashmap
 (ses-sym-rowcol): Use hashmap for named cell.
 (ses-is-cell-sym-p): New defun.
 (ses-decode-cell-symbol): New defun.
 (ses-create-cell-variable): Add cell to hashmap when name is not A1-like.
 (ses-rename-cell): Check that cell new name is not already in
 spreadsheet with the use of ses-is-cell-sym-p
 (ses-rename-cell): Use hash map for named cells, but accept also
 renaming back to A1-like.