Extend the ELTL parser to support basic aliases of automaton

operators such as F=U(true,$0) or R=!U(!$0,!$1), and infix
notation for binary automaton operators.

* README: Document the ELTL directories.
* src/eltlparse/eltlparse.yy, src/eltlparse/eltlscan.ll: Add
support for aliases and infix notation.
* src/eltlparse/public.hh, src/ltlast/nfa.cc, src/ltlast/nfa.hh:
Clean them.
* src/eltltest/acc.test, src/tgbatest/eltl2tgba.test: Add tests
for the ELTL parser's extensions.
* src/tgbatest/eltl2tgba.cc: Adjust.

src/ltlast/nfa.cc

@@ -27,7 +27,7 @@ namespace spot
   namespace ltl
   {
     nfa::nfa()
-      : is_(), si_(), arity_(0), name_(), init_(0), finals_()
+      : is_(), si_(), arity_(-1), name_(), init_(0), finals_()
     {
     }
 

src/ltlast/nfa.hh

@@ -102,7 +102,7 @@ namespace spot
       si_map si_;
 
       int arity_;
-      std::string name_; // FIXME.
+      std::string name_;
 
       state* init_;
       std::set<int> finals_;