* src/ltlparse/ltlscan.ll: Understand the combining overline, and
combining overbar as synonym for =0.
* src/ltlvisit/tostring.cc: Emit a combining overline for
single-letter atomic propositions.
* src/ltlast/atomic_prop.hh (is_atomic_prop): New function.
* doc/tl/tl.tex: Document these two characters.