Discard tokens after error detection to find the best repair

* Use GLR stack-splitting to try all numbers of tokens to
  discard until a repair is found.
* Check the validity of repairs by looking at the child trees,
  rather than the statically-computed 'in-progress symbols' list

src/compiler/parse_table.h

@@ -84,7 +84,6 @@ class ParseState {
   void each_advance_action(std::function<void(ParseAction *)>);
 
   std::map<rules::Symbol, std::vector<ParseAction>> actions;
-  std::set<rules::Symbol> in_progress_symbols;
   LexStateId lex_state_id;
 };