Warn about unused values.

* src/symlist.h, src/symlist.c (symbol_list, symbol_list_new): Add
a `used' member.
(symbol_list_n_get, symbol_list_n_used_set): New.
(symbol_list_n_type_name_get): Use symbol_list_n_get.
* src/scan-gram.l (handle_action_dollar): Flag used symbols.
* src/reader.c (grammar_current_rule_check): Check that values are
used.
* src/symtab.c (symbol_print): Accept 0.
* tests/existing.at: Remove the type information.
Empty the actions.
Remove useless actions (beware of mid-rule actions: perl -000
-pi -e 's/s*{}(?=[ns]*[|;])//g').
* tests/actions.at (Exotic Dollars): Use unused values.
* tests/calc.at: Likewise.
* tests/glr-regression.at (No users destructors if stack 0 deleted):
Likewise.
* src/gram.c (rule_useful_p, rule_never_reduced_p): Use
rule_useful_p.

tests/glr-regression.at

@@ -746,13 +746,14 @@ AT_SETUP([No users destructors if stack 0 deleted])
 AT_DATA_GRAMMAR([glr-regr9.y],
 [[
 %{
-  #include <stdio.h>
-  #include <stdlib.h>
+# include <stdio.h>
+# include <stdlib.h>
   static void yyerror (char const *);
   static int yylex (void);
-  #define YYSTACKEXPANDABLE 0
+# define YYSTACKEXPANDABLE 0
   static int tokens = 0;
   static int destructors = 0;
+# define USE(Var)
 %}
 
 %glr-parser
@@ -766,7 +767,7 @@ AT_DATA_GRAMMAR([glr-regr9.y],
 %%
 
 start:
-  ambig0 'a' { destructors += 2; }
+    ambig0 'a'   { destructors += 2; USE ($2); }
   | ambig1 start { destructors += 1; }
   | ambig2 start { destructors += 1; }
   ;