Cache decoding objects for better benchmark results. (~15%)

I would prefer to find an API that is both fast and doesn't
require this, but we'll do this for now.

benchmarks/parsetostruct.upb_table.c

@@ -10,6 +10,10 @@
 static upb_string *input_str;
 static upb_msgdef *def;
 static upb_msg *msg;
+static upb_stringsrc strsrc;
+static upb_decoder d;
+static upb_handlers h;
+static upb_msgpopulator p;
 
 static bool initialize()
 {
@@ -49,6 +53,11 @@ static bool initialize()
   }
   upb_status_uninit(&status);
   msg = upb_msg_new(def);
+
+  upb_stringsrc_init(&strsrc);
+  upb_decoder_init(&d, def);
+  upb_msgpopulator_init(&p);
+  upb_handlers_init(&h);
   return true;
 }
 
@@ -57,6 +66,10 @@ static void cleanup()
   upb_string_unref(input_str);
   upb_msg_unref(msg, def);
   upb_def_unref(UPB_UPCAST(def));
+  upb_stringsrc_uninit(&strsrc);
+  upb_decoder_uninit(&d);
+  upb_msgpopulator_uninit(&p);
+  upb_handlers_uninit(&h);
 }
 
 static size_t run(int i)
@@ -64,7 +77,15 @@ static size_t run(int i)
   (void)i;
   upb_status status = UPB_STATUS_INIT;
   upb_msg_clear(msg, def);
-  upb_strtomsg(input_str, msg, def, &status);
+  upb_stringsrc_reset(&strsrc, input_str);
+  upb_decoder_reset(&d, upb_stringsrc_bytesrc(&strsrc));
+  upb_msgpopulator_reset(&p, msg, def);
+  upb_handlers_init(&h);
+  upb_msgpopulator_register_handlers(&p, &h);
+  upb_src *src = upb_decoder_src(&d);
+  upb_src_sethandlers(src, &h);
+
+  upb_src_run(src, &status);
   if(!upb_ok(&status)) goto err;
   upb_status_uninit(&status);
   return upb_string_len(input_str);