* src/symtab.h, src/symtab.c (symbol_t): printer and

* src/symtab.h, src/symtab.c (symbol_t): printer and
printer_location are new members.
(symbol_printer_set): New.
* src/parse-gram.y (PERCENT_PRINTER): New token.
Handle its associated rule.
* src/scan-gram.l: Adjust.
(handle_destructor_at, handle_destructor_dollar): Rename as...
(handle_symbol_code_at, handle_symbol_code_dollar): these.
* src/output.c (symbol_printers_output): New.
(output_skeleton): Call it.
* data/bison.simple (yysymprint): New.  Cannot be named yyprint
since there are already many grammar files with a user `yyprint'.
Replace the calls to YYPRINT to calls to yysymprint.
* tests/calc.at: Adjust.
* tests/torture.at (AT_DATA_STACK_TORTURE): Remove YYPRINT: it was
taking advantage of parser very internal details (stack size!).