spot::ltl:: prefixes.
(to_string(formula)): New function.
* src/ltlvisit/tostring.hh (to_string(formula)): Likewise.
* src/ltltest/tostring.cc: Use this new to_string function to
simplify.
* src/ltltest/tostring.cc: New files.
* src/ltlvisit/tostring.hh: From ast to string New files.
* src/ltlvisit/tostring.cc: From ast to string New files.