Finish implementation of per-type %destructor/%printer. Discussed

starting at
<http://lists.gnu.org/archive/html/bison-patches/2006-02/msg00064.html>
and
<http://lists.gnu.org/archive/html/bison-patches/2006-06/msg00091.html>.
* NEWS (2.3+): Add a description of this feature to the default
%destructor/%printer description.
* doc/bison.texinfo (Freeing Discarded Symbols): Likewise.
* src/symlist.c (symbol_list_destructor_set, symbol_list_printer_set):
Invoke semantic_type_destructor_set or semantic_type_printer_set when a
list node contains a semantic type.
* src/symtab.c, src/symtab.h: Extend with a table that associates
semantic types with their %destructor's and %printer's.
(semantic_type_from_uniqstr, semantic_type_get,
semantic_type_destructor_set, semantic_type_printer_set): New functions
composing the public interface of that table.
(symbol_destructor_get, symbol_destructor_location_get,
symbol_printer_get, symbol_printer_location_get):  If there's no
per-symbol %destructor/%printer, look up the per-type before trying
the default.
* tests/actions.at (Per-type %printer and %destructor): New test case.
* tests/input.at (Default %printer and %destructor redeclared):
Extend to check that multiple occurrences of %symbol-default in a
single %destructor/%printer declaration is an error.
(Per-type %printer and %destructor redeclared, Unused values with
per-type %destructor): New test cases.

doc/bison.texinfo

@@ -4011,19 +4011,24 @@ Within @var{code}, @code{$$} designates the semantic value associated
 with the discarded symbol, and @code{@@$} designates its location.
 The additional parser parameters are also available (@pxref{Parser Function, ,
 The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you may define a default @code{%destructor} by placing
+@code{%symbol-default} in the @var{symbols} list of exactly one
+@code{%destructor} declaration in your grammar file.
+In that case, the parser will invoke the associated @var{code} whenever it
+discards any user-defined grammar symbol for which there is no per-type or
+per-symbol @code{%destructor}.
 @end deffn
 
-@deffn {Directive} %destructor @{ @var{code} @} %symbol-default
-@cindex default %destructor
-@findex %symbol-default
-Invoke the braced @var{code} whenever the parser discards any user-defined
-grammar symbol for which the user has not specifically declared any
-@code{%destructor}.
-This is known as the default @code{%destructor}.
-As in the previous form, @code{$$}, @code{@@$}, and the additional parser
-parameters are available.
-@end deffn
-
+@noindent
 For instance:
 
 @smallexample
@@ -4032,13 +4037,18 @@ For instance:
 %token <string> STRING2
 %type  <string> string1
 %type  <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type  <character> chr
 %destructor @{ free ($$); @} %symbol-default
 %destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ @} <character>
 @end smallexample
 
 @noindent
-guarantees that, when the parser discards any user-defined symbol, it passes
-its semantic value to @code{free}.
+guarantees that, when the parser discards any user-defined symbol that has a
+semantic type tag other than @code{<character>}, it passes its semantic value
+to @code{free}.
 However, when the parser discards a @code{STRING1} or a @code{string1}, it also
 prints its line number to @code{stdout}.
 It performs only the second @code{%destructor} in this case, so it invokes