v2.1

• Add ltl2ba_version() + macros to query link/compile-time version of library.
• Adapt C output for level of support of upcoming ESBMC release v7.6.
• Parse U, V and -> as right-associative operators and forbid chaining <->
  because there is no universally agreed upon interpretation of a <-> b <-> c.
• Parser checks that the entire formula was parsed (it silently allowed
  'a X b' before, interpreting just 'a').