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.c

@@ -482,6 +482,12 @@ state_item_print (const state_item *si, FILE *out, const char *prefix)
   putc ('\n', out);
 }
 
+const rule*
+state_item_rule (const state_item *si)
+{
+  return item_rule (si->item);
+}
+
 /**
  * Report the state_item graph
  */