Remove the separator characters construct

Now, grammars can handle whitespace by making it another ubiquitous
token, like comments.

For now, this has the side effect of whitespace being included in the
tree that precedes it. This was already an issue for other ubiquitous
tokens though, so it needs to be fixed anyway.

src/compiler/generate_code/c_code.cc

@@ -314,8 +314,6 @@ class CCodeGenerator {
 
   void code_for_lex_state(const LexState &lex_state) {
     auto expected_inputs = lex_state.expected_inputs();
-    if (lex_state.is_token_start)
-      line("START_TOKEN();");
     for (auto pair : lex_state.actions)
       if (!pair.first.is_empty())
         _if([&]() { condition_for_character_set(pair.first); },