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.

doc/bison.texi

@@ -9937,9 +9937,9 @@ output is actually in color)}:
 Shift/reduce conflict on token "else":
 @group
   Example            @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
-  First derivation   @yellow{if_stmt ::=[ "if" expr "then"} @green{stmt ::=[} @blue{if_stmt ::=[ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
+  First derivation   @yellow{if_stmt @arrow [ "if" expr "then"} @green{stmt @arrow [} @blue{if_stmt @arrow [ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
   Example            @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @blue{"else" stmt}
-  Second derivation  @yellow{if_stmt ::=[ "if" expr "then"} @green{stmt ::=[} @blue{if_stmt ::=[ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
+  Second derivation  @yellow{if_stmt @arrow [ "if" expr "then"} @green{stmt @arrow [} @blue{if_stmt @arrow [ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
 @end group
 @end example
 @end ifhtml
@@ -9950,11 +9950,11 @@ Shift/reduce conflict on token "else":
   Example
     @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @yellow{"else" stmt}
   First derivation
-    @yellow{if_stmt ::=[ "if" expr "then"} @green{stmt ::=[} @blue{if_stmt ::=[ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
+    @yellow{if_stmt @arrow [ "if" expr "then"} @green{stmt @arrow [} @blue{if_stmt @arrow [ "if" expr "then" stmt} @red{•} @blue{]} @green{]} @yellow{"else" stmt ]}
   Example
     @yellow{"if" expr "then"} @blue{"if" expr "then" stmt} @red{•} @blue{"else" stmt}
   Second derivation
-    @yellow{if_stmt ::=[ "if" expr "then"} @green{stmt ::=[} @blue{if_stmt ::=[ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
+    @yellow{if_stmt @arrow [ "if" expr "then"} @green{stmt @arrow [} @blue{if_stmt @arrow [ "if" expr "then" stmt} @red{•} @blue{"else" stmt ]} @green{]} @yellow{]}
 @end group
 @end smallexample
 @end ifnothtml
@@ -9988,21 +9988,21 @@ sequence.y: @dwarning{warning}: 1 shift/reduce conflict [@dwarning{-Wconflicts-s
 sequence.y: @dwarning{warning}: 2 reduce/reduce conflicts [@dwarning{-Wconflicts-rr}]
 Shift/reduce conflict on token "word":
   Example              @red{•} @yellow{"word"}
-  First derivation     @yellow{sequence ::=[} @green{sequence ::=[} @red{•} @green{]} @yellow{"word" ]}
+  First derivation     @yellow{sequence @arrow [} @green{sequence @arrow [} @red{•} @green{]} @yellow{"word" ]}
   Example              @red{•} @green{"word"}
-  Second derivation    @yellow{sequence ::=[} @green{maybeword ::=[} @red{•} @green{"word" ]} @yellow{]}
+  Second derivation    @yellow{sequence @arrow [} @green{maybeword @arrow [} @red{•} @green{"word" ]} @yellow{]}
 
 Reduce/reduce conflict on tokens $end, "word":
   Example              @red{•}
-  First derivation     @yellow{sequence ::=[} @red{•} @yellow{]}
+  First derivation     @yellow{sequence @arrow [} @red{•} @yellow{]}
   Example              @red{•}
-  Second derivation    @yellow{sequence ::=[} @green{maybeword ::=[} @red{•} @green{]} @yellow{]}
+  Second derivation    @yellow{sequence @arrow [} @green{maybeword @arrow [} @red{•} @green{]} @yellow{]}
 
 Shift/reduce conflict on token "word":
   Example              @red{•} @yellow{"word"}
-  First derivation     @yellow{sequence ::=[} @green{sequence ::=[} @blue{maybeword ::=[} @red{•} @blue{]} @green{]} @yellow{"word" ]}
+  First derivation     @yellow{sequence @arrow [} @green{sequence @arrow [} @blue{maybeword @arrow [} @red{•} @blue{]} @green{]} @yellow{"word" ]}
   Example              @red{•} @green{"word"}
-  Second derivation    @yellow{sequence ::=[} @green{maybeword ::=[} @red{•} @green{"word" ]} @yellow{]}
+  Second derivation    @yellow{sequence @arrow [} @green{maybeword @arrow [} @red{•} @green{"word" ]} @yellow{]}
 
 sequence.y:8.3-45: @dwarning{warning}: rule useless in parser due to conflicts [@dwarning{-Wother}]
     8 |   @dwarning{%empty    @{ printf ("empty maybeword\n"); @}}
@@ -10034,9 +10034,9 @@ expr: %empty | expr ID ','
 @smallexample
 Shift/reduce conflict on token ID:
   First example        @blue{expr} @red{•} @green{ID} @yellow{$end}
-  First derivation     @yellow{$accept ::=[} @green{s ::=[} @blue{a ::=[ expr} @red{•} @blue{]} @green{ID ]} @yellow{$end ]}
+  First derivation     @yellow{$accept @arrow [} @green{s @arrow [} @blue{a @arrow [ expr} @red{•} @blue{]} @green{ID ]} @yellow{$end ]}
   Second example       @purple{expr} @red{•} @purple{ID ','} @green{ID} @yellow{$end}
-  Second derivation    @yellow{$accept ::=[} @green{s ::=[} @blue{a ::=[} @purple{expr ::=[ expr} @red{•} @purple{ID ',' ]} @blue{]} @green{ID ]} @yellow{$end ]}
+  Second derivation    @yellow{$accept @arrow [} @green{s @arrow [} @blue{a @arrow [} @purple{expr @arrow [ expr} @red{•} @purple{ID ',' ]} @blue{]} @green{ID ]} @yellow{$end ]}
 @end smallexample
 
 This conflict is caused by the parser not having enough information to know
@@ -10433,9 +10433,9 @@ Shift/reduce conflict on token '/':
     1 exp: exp '+' exp •
     4 exp: exp • '/' exp
   Example              @green{exp '+' exp} @red{•} @yellow{'/' exp}
-  First derivation     @yellow{exp ::=[} @green{exp ::=[ exp '+' exp} @red{•} @green{]} @yellow{'/' exp ]}
+  First derivation     @yellow{exp @arrow [} @green{exp @arrow [ exp '+' exp} @red{•} @green{]} @yellow{'/' exp ]}
   Example              @yellow{exp '+'} @green{exp} @red{•} @green{'/' exp}
-  Second derivation    @yellow{exp ::=[ exp '+'} @green{exp ::=[ exp} @red{•} @green{'/' exp ]} @yellow{]}
+  Second derivation    @yellow{exp @arrow [ exp '+'} @green{exp @arrow [ exp} @red{•} @green{'/' exp ]} @yellow{]}
 @end example
 
 This shows two separate derivations in the grammar for the same @code{exp}: