cex: also include the counterexamples in the report

The report is the best place to show the details about
counterexamples, since we have the state right under the nose.

For instance:

State 7

    1 exp: exp . "⊕" exp
    2    | exp . "+" exp
    2    | exp "+" exp .  [$end, "+", "⊕"]
    3    | exp . "+" exp
    3    | exp "+" exp .  [$end, "+", "⊕"]

    "⊕"  shift, and go to state 6

    $end      reduce using rule 2 (exp)
    $end      [reduce using rule 3 (exp)]
    "+"       reduce using rule 2 (exp)
    "+"       [reduce using rule 3 (exp)]
    "⊕"       [reduce using rule 2 (exp)]
    "⊕"       [reduce using rule 3 (exp)]
    $default  reduce using rule 2 (exp)

    Conflict between rule 2 and token "+" resolved as reduce (%left "+").

    Shift/reduce conflict on token "⊕":
        2 exp: exp "+" exp .
        1 exp: exp . "⊕" exp
      Example                  exp "+" exp • "⊕" exp
      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
      Example                  exp "+" exp • "⊕" exp
      Second derivation        exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]

    Reduce/reduce conflict on tokens $end, "+", "⊕":
        2 exp: exp "+" exp .
        3 exp: exp "+" exp .
      Example                  exp "+" exp •
      First derivation         exp ::=[ exp "+" exp • ]
      Example                  exp "+" exp •
      Second derivation        exp ::=[ exp "+" exp • ]

    Shift/reduce conflict on token "⊕":
        3 exp: exp "+" exp .
        1 exp: exp . "⊕" exp
      Example                  exp "+" exp • "⊕" exp
      First derivation         exp ::=[ exp ::=[ exp "+" exp • ] "⊕" exp ]
      Example                  exp "+" exp • "⊕" exp
      Second derivation        exp ::=[ exp "+" exp ::=[ exp • "⊕" exp ] ]

* src/conflicts.h, src/conflicts.c (has_conflicts): New.
* src/counterexample.h, src/counterexample.c (print_counterexample):
Add a `prefix` argument.
(counterexample_report_shift_reduce)
(counterexample_report_reduce_reduce): Show the items when there's a
prefix.
* src/state-item.h, src/state-item.c (print_state_item):
Add a `prefix` argument.
* src/derivation.h, src/derivation.c (derivation_print)
(derivation_print_leaves): Add a prefix argument.
* src/print.c (print_state): When -Wcex is enabled, show the
conflicts.
* tests/report.at: Adjust.

TODO

@@ -1,5 +1,6 @@
 * Bison 3.7
 ** Cex
+*** Improve gnulib
 Don't do this (counterexample.c):
 
 // This is the fastest way to get the tail node from the gl_list API.
@@ -12,6 +13,25 @@ list_get_end (gl_list_t list)
   return res;
 }
 
+*** Ambiguous rewriting
+If the user is stupid enough to have equal rules, then the derivations are
+harder to read:
+
+    Reduce/reduce conflict on tokens $end, "+", "⊕":
+        2 exp: exp "+" exp .
+        3 exp: exp "+" exp .
+      Example                  exp "+" exp •
+      First derivation         exp ::=[ exp "+" exp • ]
+      Example                  exp "+" exp •
+      Second derivation        exp ::=[ exp "+" exp • ]
+
+Do we care about this?  In color, we use twice the same color here, but we
+could try to use the same color for the same rule.
+
+*** XML reports
+Show the counterexamples.  This is going to be really hard and/or painful.
+Unless we play it dumb (little structure).
+
 ** glr.cc
 Get rid of global_tokens_and_yystype.