Major updates/cleanup.

include/internal/parser/parser.h

@@ -93,6 +93,9 @@ struct Parser {
     std::vector<io::Token> *tokens, CompletionContext *completion
   );
 
+  void token(io::Token::Kind);
+  void end_token();
+
   char peek(uint16_t = 0); // == \0 if at eof.
   std::string_view peek_str(uint16_t = UINT16_MAX);
   void advance(uint16_t = 1);