Enable addition of local printers from a mode hook.

Enable addition of local printers from a mode hook.

* doc/misc/ses.texi (Printer functions): Split the node into 5
sub-nodes + add some extra documentation.
(Various kinds of printer functions): Make an itemisation to
disintguish better the 3 types of printers, give an example of
lambda printer definition.
(Standard printer functions): Add documentation for ses-prin1
printer function.
(Local printer functions): Add documentation for creating
local printers programmatically from a hook.
(Writing a lambda printer function): Add documentation about
anti-stackoverflow precautions to take when you call the
standard printer functions from inside a local printer.

* lisp/ses.el (ses-standard-printer-functions): Add ses-prin1
among standard printer function, and update docstring
accordingly.
(ses-call-printer, ses-export-tab): Call `ses-prin1' instead
of prin1-to-string.
(ses-define-local-printer): Add definition to arguments so
that a local printer can be defined programmatically from a
mode hook.  Make docstring more substantial.  Use completing
read for local printer name input.  Plus some minor
optimization.
(ses-define-if-new-local-printer): New defsubst.
(ses-center, ses-center-span, ses-dashfill)
(ses-dashfill-span, ses-tildefill-span): Allow to pass printer
as an optional argument to superseed column printer/default
spreadsheet printer.
(ses-prin1): New defun.