cex: display the rule numbers

From

    Example: "if" expr "then" "if" expr "then" stmt • "else" stmt
    Shift derivation
      if_stmt
      ↳ "if" expr "then" stmt
                         ↳ if_stmt
                           ↳ "if" expr "then" stmt • "else" stmt
    Reduce derivation
      if_stmt
      ↳ "if" expr "then" stmt                        "else" stmt
                         ↳ if_stmt
                           ↳ "if" expr "then" stmt •

to

    Example: "if" expr "then" "if" expr "then" stmt • "else" stmt
    Shift derivation
      if_stmt
      ↳ 3: "if" expr "then" stmt
                            ↳ 2: if_stmt
                                 ↳ 4: "if" expr "then" stmt • "else" stmt
    Example: "if" expr "then" "if" expr "then" stmt • "else" stmt
    Reduce derivation
      if_stmt
      ↳ 4: "if" expr "then" stmt                              "else" stmt
                            ↳ 2: if_stmt
                                 ↳ 3: "if" expr "then" stmt •

* src/state-item.h, src/state-item.c (state_item_rule): New.
* src/derivation.h, src/derivation.c (struct derivation): Add a rule
member.
Adjust dependencies.
* src/counterexample.c, src/parse-simulation.c: Pass the rule to
derivation_new.
* src/derivation.c (fprintf_if): New.
(derivation_width, derivation_print_tree_impl): Take the rule number
into account.

* tests/conflicts.at, tests/counterexample.at, tests/diagnostics.at,
* tests/report.at: Adjust.

* doc/bison.texi: Adjust.

src/state-item.h

@@ -93,6 +93,7 @@ void state_items_init (void);
 void state_items_free (void);
 
 void state_item_print (const state_item *si, FILE *out, const char *prefix);
+const rule *state_item_rule (const state_item *si);
 
 bool production_allowed (const state_item *si, const state_item *next);