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/calc.at

@@ -54,6 +54,7 @@ AT_LALR1_CC_IF(
 # define alarm(seconds) /* empty */
 #endif
 #include <ctype.h>
+#define USE(Var)
 
 /* Exercise pre-prologue dependency to %union.  */
 typedef int semantic_value;
@@ -109,7 +110,7 @@ input:
 
 line:
   '\n'
-| exp '\n'           { ]AT_PARAM_IF([*result = global_result = $1;])[ }
+| exp '\n'           { ]AT_PARAM_IF([*result = global_result = $1], [USE ($1)])[; }
 ;
 
 exp: