Revert "Remove the separator characters construct"

This reverts commit 5cd07648fd.

The separators construct is useful as an optimization. It turns out that
constructing a node for every chunk of whitespace in a document causes a
significant performance regression.

Conflicts:
	src/compiler/build_tables/build_lex_table.cc
	src/compiler/grammar.cc
	src/runtime/parser.c

src/compiler/generate_code/c_code.cc

@@ -314,6 +314,8 @@ 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); },