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/eltltest/acc.test

@@ -33,7 +33,24 @@ U=(
     0 1 \$1
     accept 1
   )
+F=U(true, \$0)
+R=!U(!\$0, !\$1)
+%
+a U b | a R b | F(true) | U(a,b) -> R(a,b)
 EOF
+run 0 ./acc input || exit 1
+
+cat >input <<EOF
+U=(
+    0 0 \$0
+    0 1 \$1
+    accept 1
+  )
+T=U(true,true)
+%
+T()|1
+EOF
+run 1 ./acc input || exit 1
 
 cat >input <<EOF
 A=(