Refactor visitors into separate files

This also includes some small improvements that I don't remember right now

src/meson.build

@@ -2,6 +2,8 @@ executable(meson.project_name(),
   'main.cpp',
   'parser.cpp',
   'split.cpp',
+  'visitors/debug_visitor.cpp',
+  'visitors/find_t_visitor.cpp',
   dependencies: [
 	boost_dep,
     libstriezel_dep,