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.

NEWS

@@ -27,6 +27,19 @@ New in spot 2.5.2.dev (not yet released)
     simplified to {1} or {SERE} depending on whether SERE accepts
     the empty word or not.
 
+  - gf_guarantee_to_ba() is a specialized construction for
+    translating formulas of the form GF(guarantee) to BA or DBA,
+    and fg_safety_to_dca() is a specialized construction for
+    translating formulas of the form FG(safety) to DCA.  These
+    are slight generalizations of some constructions proposed
+    by J. Esparza, J. Křentínský, and S. Sickert in a submitted
+    paper.
+
+    These are now used by the main translation routine, and can be
+    disabled by passing -x '!gf-guarantee' to ltl2tgba.  As an
+    example, the translation of GF(a <-> XXb) to transition-based
+    Büchi went from 9 to 5 states using that construction.
+
   Bugs fixed:
 
   - "autfilt --cobuchi --small/--det" would turn a transition-based