We should write .sig files for libraries.

This command is maybe too simple but it shows that we are missing some of them.

$ cd /path/to/HolBA/src/
$ find . -name "*Lib.sml" -exec sh -c "echo {} | sed 's/Lib.sml/Lib.sig/' | xargs ls >/dev/null" \;
ls: cannot access './tools/cfg/graphVizLib.sig': No such file or directory
ls: cannot access './tools/cfg/bir_cfgLib.sig': No such file or