specialized translation for GF(guarantee) and FG(safety)

This is adapted from a proposition in a paper by J. Esparza,
J. Křentínský, and S. Sickert, submitted to LICS'18.  We should add
proper references to the code and documentation once that paper is
accepted.

* spot/twaalgos/gfguarantee.cc, spot/twaalgos/gfguarantee.hh:
New files.
* spot/twaalgos/Makefile.am, python/spot/impl.i: Add them.
* spot/twa/fwd.hh: Add a forward declaration of bdd_dict_ptr.
* spot/twaalgos/postproc.cc, spot/twaalgos/postproc.hh: Make it
possible to call finalize() from the translator subclass.  Constify
all the do_* functions while we are there.
* spot/twaalgos/translate.cc, spot/twaalgos/translate.hh: Add
a "gf-guarantee" option to decide whether to use the new translation.
* bin/spot-x.cc: Document it.
* tests/core/dca2.test, tests/core/genltl.test,
tests/core/ltl2tgba2.test, tests/core/parity2.test,
tests/core/satmin.test, tests/python/automata.ipynb,
tests/python/sbacc.py: Adjust test cases.
* tests/python/except.py: Add a couple more tests.

tests/core/genltl.test

@@ -150,11 +150,11 @@ ms-phi-r=2,29
 ms-phi-s=0,5
 ms-phi-s=1,8
 ms-phi-s=2,497
-ms-phi-h=0,2
-ms-phi-h=1,4
-ms-phi-h=2,21
-ms-phi-h=3,170
-ms-phi-h=4,1816
+ms-phi-h=0,1
+ms-phi-h=1,3
+ms-phi-h=2,7
+ms-phi-h=3,15
+ms-phi-h=4,31
 gf-equiv=0,1
 gf-equiv=1,4
 gf-equiv=2,8