cex: prefer → to ::=

It does not make a lot of sense to use ::= in our counterexamples,
that's not something that belongs to the Bison "vocabulary".  Using
the colon makes sense, but it's too discreet.  Let's use the arrow,
which we already use in some reports (HTML and Dot).

* src/gram.h (print_dot_fallback): Generalize into...
(print_fallback): this.
(print_arrow): New.
* src/derivation.c: Use it.

* NEWS, tests/conflicts.at, tests/counterexample.at,
* tests/diagnostics.at, tests/report.at: Adjust.
* doc/bison.texi: Ditto.
Unfortunately the literal `→` is output as `↦`.  So we need to use
@arrow.

src/derivation.c

@@ -145,7 +145,9 @@ derivation_print_impl (const derivation *deriv, FILE *f,
         {
           fputs (prefix, f);
           begin_use_class ("cex-step", f);
-          fprintf (f, "%s ::=[ ", sym->tag);
+          fprintf (f, "%s ", sym->tag);
+          print_arrow (f);
+          fprintf (f, " [ ");
           end_use_class ("cex-step", f);
           prefix = "";
         }