Use 'const formula*' instead of 'formula*' everywhere.

The distinction makes no sense since Spot 0.5, where we switched from
mutable furmulae to immutable formulae.  The difference between
const_visitor and visitor made no sense either.  They have been merged
into one: visitor.

* iface/dve2/dve2check.cc, iface/gspn/ltlgspn.cc,
src/eltlparse/eltlparse.yy, src/eltlparse/public.hh,
src/evtgbatest/ltl2evtgba.cc, src/kripkeparse/kripkeparse.yy,
src/ltlast/atomic_prop.cc, src/ltlast/atomic_prop.hh,
src/ltlast/automatop.cc, src/ltlast/automatop.hh, src/ltlast/binop.cc,
src/ltlast/binop.hh, src/ltlast/bunop.cc, src/ltlast/bunop.hh,
src/ltlast/constant.cc, src/ltlast/constant.hh, src/ltlast/formula.cc,
src/ltlast/formula.hh, src/ltlast/formula_tree.cc,
src/ltlast/formula_tree.hh, src/ltlast/multop.cc,
src/ltlast/multop.hh, src/ltlast/predecl.hh, src/ltlast/refformula.cc,
src/ltlast/refformula.hh, src/ltlast/unop.cc, src/ltlast/unop.hh,
src/ltlast/visitor.hh, src/ltlenv/declenv.cc, src/ltlenv/declenv.hh,
src/ltlenv/defaultenv.cc, src/ltlenv/defaultenv.hh,
src/ltlenv/environment.hh, src/ltlparse/ltlfile.cc,
src/ltlparse/ltlfile.hh, src/ltlparse/ltlparse.yy,
src/ltlparse/public.hh, src/ltltest/consterm.cc,
src/ltltest/equals.cc, src/ltltest/genltl.cc, src/ltltest/kind.cc,
src/ltltest/length.cc, src/ltltest/randltl.cc, src/ltltest/readltl.cc,
src/ltltest/reduc.cc, src/ltltest/syntimpl.cc,
src/ltltest/tostring.cc, src/ltlvisit/apcollect.cc,
src/ltlvisit/apcollect.hh, src/ltlvisit/clone.cc,
src/ltlvisit/clone.hh, src/ltlvisit/contain.cc,
src/ltlvisit/contain.hh, src/ltlvisit/dotty.cc,
src/ltlvisit/length.cc, src/ltlvisit/lunabbrev.cc,
src/ltlvisit/lunabbrev.hh, src/ltlvisit/mark.cc, src/ltlvisit/mark.hh,
src/ltlvisit/nenoform.cc, src/ltlvisit/nenoform.hh,
src/ltlvisit/postfix.cc, src/ltlvisit/postfix.hh,
src/ltlvisit/randomltl.cc, src/ltlvisit/randomltl.hh,
src/ltlvisit/reduce.cc, src/ltlvisit/reduce.hh,
src/ltlvisit/simpfg.cc, src/ltlvisit/simpfg.hh,
src/ltlvisit/simplify.cc, src/ltlvisit/simplify.hh,
src/ltlvisit/snf.cc, src/ltlvisit/snf.hh, src/ltlvisit/tostring.cc,
src/ltlvisit/tunabbrev.cc, src/ltlvisit/tunabbrev.hh,
src/ltlvisit/wmunabbrev.cc, src/ltlvisit/wmunabbrev.hh,
src/neverparse/neverclaimparse.yy, src/sabatest/sabacomplementtgba.cc,
src/tgba/bdddict.cc, src/tgba/formula2bdd.cc, src/tgba/taatgba.cc,
src/tgba/taatgba.hh, src/tgbaalgos/eltl2tgba_lacim.cc,
src/tgbaalgos/ltl2taa.cc, src/tgbaalgos/ltl2tgba_fm.cc,
src/tgbaalgos/ltl2tgba_lacim.cc, src/tgbaalgos/minimize.cc,
src/tgbaalgos/randomgraph.cc, src/tgbaparse/tgbaparse.yy,
src/tgbatest/complementation.cc, src/tgbatest/ltl2tgba.cc,
src/tgbatest/ltlprod.cc, src/tgbatest/mixprod.cc,
src/tgbatest/randtgba.cc: Massive adjustment!
* src/tgbatest/reductgba.cc: Delete.

NEWS

@@ -6,10 +6,6 @@ New in spot 0.8.3a:
       Expressions (SERE), and a couple of operators to bridge SERE and
       LTL.  See doc/tl/tl.pdf for the list of operators and their
       semantics.
-    - The constructors for temporal formulae will perform some trivial
-      simplifications based on associativity, commutativity,
-      idempotence, and neutral elements.  See doc/tl/tl.pdf for the
-      list of such simplifications.
     - Formula rewritings have been completely revamped, and augmented
       with rules for PSL operators (and some new LTL rules as well).
       See doc/tl/tl.pdf for the list of the rewritings implemented.
@@ -51,13 +47,24 @@ New in spot 0.8.3a:
   * Interface changes:
     - Operators ->, <->, U, W, R, and M are now parsed as
       right-associative to better match the PSL standard.
+    - The constructors for temporal formulae will perform some trivial
+      simplifications based on associativity, commutativity,
+      idempotence, and neutral elements.  See doc/tl/tl.pdf for the
+      list of such simplifications.
+    - Formula instances now have many methods to inspect their
+      properties (membership to syntactic classes, absence of X
+      operator, etc...) in constant time.
+    - LTL/PSL formulae are now handled everywhere as 'const formula*'
+      and not just 'formula*'.  This reflects the true nature of these
+      (immutable) formula objects, and cleanups a lot of code.
+      Unfortunately, it is a backward incompatible change: you may have
+      to add 'const' to a couple of lines in your code, and change
+      'ltl::const_vistitor' into 'ltl::visitor' if you have written a
+      custom visitor.
     - The new entry point for LTL/PSL simplifications is the function
       ltl_simplifier::simplify() declared in src/ltlvisit/simplify.hh.
       The ltl_simplifier class implements a cache.
       Functions such as reduce() or reduce_tau03() are deprecated.
-    - Formula instances now have many methods to inspect their
-      properties (membership to syntactic classes, absence of X
-      operator, etc...) in constant time.
     - The old game-theory-based implementations for direct and delayed
       simulation reductions have been removed.  The old direct
       simulation would only work on degeneralized automata, and yet