This actually performs three related changes, but separating them
would be quite inconvenient.

1) rename tostring.hh to print.hh a welcome side-effect is that
I could fix several files that included this file for not reason.

2) de-overload some of the to_string functions, and rename them
as follow:

  to_string -> print_psl, print_sere, str_psl, str_sere
  to_utf8_string -> print_utf8_psl, print_utf8_sere,
                    str_utf8_psl, str_utf8_sere
  to_spin_string -> print_spin_ltl, str_spin_ltl
  to_wring_string -> print_wring_ltl, str_wing_ltl
  to_lbt_string -> print_lbt_ltl, str_lbt_ltl
  to_latex_string -> print_latex_psl, str_latex_psl
  to_sclatex_string -> print_sclatex_psl, str_sclatex_psl

Now it is clearer what these functions do, and their restrictions.

3) all those print_* functions now take the stream to write onto
as their first argument.  This fixes #88.

* src/ltlvisit/tostring.cc, src/ltlvisit/tostring.hh: Rename into...
* src/ltlvisit/print.cc, src/ltlvisit/print.hh: ... those, and make
the changes listed above.
* doc/org/tut01.org, src/bin/common_output.cc,
src/bin/common_trans.cc, src/bin/ltl2tgba.cc, src/bin/ltl2tgta.cc,
src/bin/ltlcross.cc, src/bin/ltldo.cc, src/bin/ltlfilt.cc,
src/bin/randltl.cc, src/ltlparse/ltlparse.yy,
src/ltlvisit/Makefile.am, src/ltlvisit/mark.cc,
src/ltlvisit/relabel.cc, src/ltlvisit/simplify.cc,
src/ltlvisit/snf.cc, src/ta/taexplicit.cc, src/ta/tgtaexplicit.cc,
src/taalgos/tgba2ta.cc, src/tests/equalsf.cc, src/tests/ltl2tgba.cc,
src/tests/ltlrel.cc, src/tests/randtgba.cc, src/tests/reduc.cc,
src/tests/syntimpl.cc, src/tests/tostring.cc, src/twa/bdddict.cc,
src/twa/bddprint.cc, src/twa/taatgba.cc, src/twa/taatgba.hh,
src/twa/twagraph.cc, src/twaalgos/compsusp.cc, src/twaalgos/lbtt.cc,
src/twaalgos/ltl2taa.cc, src/twaalgos/ltl2tgba_fm.cc,
src/twaalgos/neverclaim.cc, src/twaalgos/remprop.cc,
src/twaalgos/stats.cc, wrap/python/ajax/spot.in, wrap/python/spot.py,
wrap/python/spot_impl.i: Adjust.