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.

NEWS

@@ -45,8 +45,8 @@ GNU Bison NEWS
   conflict.  For example:
 
     Example              exp '+' exp • '/' exp
-    First derivation     exp ::=[ exp ::=[ exp '+' exp • ] '/' exp ]
-    Second derivation    exp ::=[ exp '+' exp ::=[ exp • '/' exp ] ]
+    First derivation     exp → [ exp → [ exp '+' exp • ] '/' exp ]
+    Second derivation    exp → [ exp '+' exp → [ exp • '/' exp ] ]
 
   When Bison is installed with text styling enabled, the example is actually
   shown twice, with colors highlighting the ambiguity.
@@ -57,9 +57,9 @@ GNU Bison NEWS
   generates two examples that are the same up until the dot:
 
     First example        expr • ID $end
-    First derivation     $accept ::=[ s ::=[ a ::=[ expr • ] ID ] $end ]
+    First derivation     $accept → [ s → [ a → [ expr • ] ID ] $end ]
     Second example       expr • ID ',' ID $end
-    Second derivation    $accept ::=[ s ::=[ a ::=[ expr ::=[ expr • ID ',' ] ] ID ] $end ]
+    Second derivation    $accept → [ s → [ a → [ expr → [ expr • ID ',' ] ] ID ] $end ]
 
   In these cases, the parser usually doesn't have enough lookahead to
   differentiate the two given examples.