add of_str64_t string type

We need a 64-byte string for the debug_counter extension. Since adding a new
string type is so much work, the new type is named generically so that any
extension can use it.
diff --git a/c_gen/templates/loci_show.h b/c_gen/templates/loci_show.h
index 5384837..4389f4c 100644
--- a/c_gen/templates/loci_show.h
+++ b/c_gen/templates/loci_show.h
@@ -158,6 +158,7 @@
 #define LOCI_SHOW_tab_name(writer, cookie, val) LOCI_SHOW_string(writer, cookie, val)
 #define LOCI_SHOW_desc_str(writer, cookie, val) LOCI_SHOW_string(writer, cookie, val)
 #define LOCI_SHOW_ser_num(writer, cookie, val) LOCI_SHOW_string(writer, cookie, val)
+#define LOCI_SHOW_str64(writer, cookie, val) LOCI_SHOW_string(writer, cookie, val)
 
 int loci_show_match(loci_writer_f writer, void *cookie, of_match_t *match);
 #define LOCI_SHOW_match(writer, cookie, val) loci_show_match(writer, cookie, &val)