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.

tests/conflicts.at

@@ -865,8 +865,8 @@ State 5
         1 exp: exp OP exp .
         1 exp: exp . OP exp
       Example                  exp OP exp . OP exp
-      First derivation         exp ::=[ exp ::=[ exp OP exp . ] OP exp ]
-      Second derivation        exp ::=[ exp OP exp ::=[ exp . OP exp ] ]
+      First derivation         exp -> [ exp -> [ exp OP exp . ] OP exp ]
+      Second derivation        exp -> [ exp OP exp -> [ exp . OP exp ] ]
 
 ]])
 
@@ -1207,8 +1207,8 @@ State 1
         3 num: '0' .
         4 id: '0' .
       Example                  '0' .
-      First derivation         exp ::=[ num ::=[ '0' . ] ]
-      Second derivation        exp ::=[ id ::=[ '0' . ] ]
+      First derivation         exp -> [ num -> [ '0' . ] ]
+      Second derivation        exp -> [ id -> [ '0' . ] ]
 
 
 
@@ -1754,17 +1754,17 @@ State 4
        10 reported_conflicts: . %empty
         8 reported_conflicts: . 'a'
       First example            resolved_conflict . 'a'
-      First derivation         start ::=[ resolved_conflict reported_conflicts ::=[ . ] 'a' ]
+      First derivation         start -> [ resolved_conflict reported_conflicts -> [ . ] 'a' ]
       Second example           resolved_conflict . 'a' 'a'
-      Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ . 'a' ] 'a' ]
+      Second derivation        start -> [ resolved_conflict reported_conflicts -> [ . 'a' ] 'a' ]
 
     Shift/reduce conflict on token 'a':
        10 reported_conflicts: . %empty
         9 reported_conflicts: . 'a'
       First example            resolved_conflict . 'a'
-      First derivation         start ::=[ resolved_conflict reported_conflicts ::=[ . ] 'a' ]
+      First derivation         start -> [ resolved_conflict reported_conflicts -> [ . ] 'a' ]
       Second example           resolved_conflict . 'a' 'a'
-      Second derivation        start ::=[ resolved_conflict reported_conflicts ::=[ . 'a' ] 'a' ]
+      Second derivation        start -> [ resolved_conflict reported_conflicts -> [ . 'a' ] 'a' ]
 
 
 
@@ -1781,8 +1781,8 @@ State 5
         8 reported_conflicts: 'a' .
         9 reported_conflicts: 'a' .
       Example                  'a' .
-      First derivation         reported_conflicts ::=[ 'a' . ]
-      Second derivation        reported_conflicts ::=[ 'a' . ]
+      First derivation         reported_conflicts -> [ 'a' . ]
+      Second derivation        reported_conflicts -> [ 'a' . ]
 
 
 
@@ -1965,8 +1965,8 @@ AT_CHECK([[cat input.output | sed -n '/^State 0$/,/^State 1$/p']], 0,
        12 empty_c2: . %empty
        13 empty_c3: . %empty
       Example                  . 'c'
-      First derivation         start ::=[ empty_c2 ::=[ . ] 'c' ]
-      Second derivation        start ::=[ empty_c3 ::=[ . ] 'c' ]
+      First derivation         start -> [ empty_c2 -> [ . ] 'c' ]
+      Second derivation        start -> [ empty_c3 -> [ . ] 'c' ]