Add a visitor to relabel the atomic proposition in formulas.

* src/ltlvisit/relabel.cc, src/ltlvisit/relabel.hh: New files.
* src/ltlvisit/Makefile.am: Add them.
* src/ltlvisit/clone.cc (recurse): Don't call clone(), nobody
needs that.  Instead, really recurse.
* src/bin/ltlfilt.cc: Add a --relabel option.
* src/bin/genltl.cc: Relabel formulas if --lbt is used.
* src/sanity/style.test: Tweak detection of i++.

src/sanity/style.test

@@ -168,7 +168,7 @@ for dir in "${INCDIR-..}" "${INCDIR-..}"/../iface; do
       egrep '(->|[.])size\(\) [=!]= 0|![a-zA-Z0-9_]*(->|[.])size\(\)|(if |while |assert)\([a-zA-Z0-9_]*(->|[.])size\(\)\)' $tmp &&
 	diag 'Prefer empty() to check emptiness.'
 
-      egrep '^[^=*]*([+][+]|--);' $tmp &&
+      egrep '^[^=*<]*([+][+]|--);' $tmp &&
 	diag 'Take good habits: use ++i instead of i++ when you have the choice.'
 
       grep '[^a-zA-Z0-9_](\*[a-zA-Z0-9_]*)\.' $tmp &&