greatly enhance performance of json_object_object_to_json_string()

This function and its helpers used to call sprintbuf() for constant
strings. This requires a lot of performance, because the call ends
up in vprintf(), which is not necessary as there is no modification
of the string to be done.

In some other places of the code, printbuf_memappend() was used in
such cases. This is the right route to follow, because it greatly
decreases processing time.

This patch does this replacement and keeps using sprintbuf() only
where it really is required. In my testing, this resulted in a very
noticable performance improvement (n *times* faster, where n
depends on the details of what is formatted and with which options).

json_object.c

@@ -330,42 +330,42 @@ static int json_object_object_to_json_string(struct json_object* jso,
 	int had_children = 0;
 	struct json_object_iter iter;
 
-	sprintbuf(pb, "{" /*}*/);
+	printbuf_memappend(pb, "{" /*}*/, 1);
 	if (flags & JSON_C_TO_STRING_PRETTY)
-		sprintbuf(pb, "\n");
+		printbuf_memappend(pb, "\n", 1);
 	json_object_object_foreachC(jso, iter)
 	{
 		if (had_children)
 		{
-			sprintbuf(pb, ",");
+			printbuf_memappend(pb, ",", 1);
 			if (flags & JSON_C_TO_STRING_PRETTY)
-				sprintbuf(pb, "\n");
+				printbuf_memappend(pb, "\n", 1);
 		}
 		had_children = 1;
 		if (flags & JSON_C_TO_STRING_SPACED)
-			sprintbuf(pb, " ");
+			printbuf_memappend(pb, " ", 1);
 		indent(pb, level+1, flags);
-		sprintbuf(pb, "\"");
+		printbuf_memappend(pb, "\"", 1);
 		json_escape_str(pb, iter.key, strlen(iter.key));
 		if (flags & JSON_C_TO_STRING_SPACED)
-			sprintbuf(pb, "\": ");
+			printbuf_memappend(pb, "\": ", 3);
 		else
-			sprintbuf(pb, "\":");
+			printbuf_memappend(pb, "\":", 2);
 		if(iter.val == NULL)
-			sprintbuf(pb, "null");
+			printbuf_memappend(pb, "null", 4);
 		else
 			iter.val->_to_json_string(iter.val, pb, level+1,flags);
 	}
 	if (flags & JSON_C_TO_STRING_PRETTY)
 	{
 		if (had_children)
-			sprintbuf(pb, "\n");
+			printbuf_memappend(pb, "\n",1);
 		indent(pb,level,flags);
 	}
 	if (flags & JSON_C_TO_STRING_SPACED)
-		return sprintbuf(pb, /*{*/ " }");
+		return printbuf_memappend(pb, /*{*/ " }", 2);
 	else
-		return sprintbuf(pb, /*{*/ "}");
+		return printbuf_memappend(pb, /*{*/ "}", 1);
 }
 
 
@@ -504,9 +504,9 @@ static int json_object_boolean_to_json_string(struct json_object* jso,
 						  int flags)
 {
 	if (jso->o.c_boolean)
-		return sprintbuf(pb, "true");
+		return printbuf_memappend(pb, "true", 4);
 	else
-		return sprintbuf(pb, "false");
+		return printbuf_memappend(pb, "false", 5);
 }
 
 struct json_object* json_object_new_boolean(json_bool b)
@@ -772,9 +772,9 @@ static int json_object_string_to_json_string(struct json_object* jso,
 					     int level,
 						 int flags)
 {
-	sprintbuf(pb, "\"");
+	printbuf_memappend(pb, "\"", 1);
 	json_escape_str(pb, get_string_component(jso), jso->o.c_string.len);
-	sprintbuf(pb, "\"");
+	printbuf_memappend(pb, "\"", 1);
 	return 0;
 }
 
@@ -869,39 +869,39 @@ static int json_object_array_to_json_string(struct json_object* jso,
 {
 	int had_children = 0;
 	int ii;
-	sprintbuf(pb, "[");
+	printbuf_memappend(pb, "[", 1);
 	if (flags & JSON_C_TO_STRING_PRETTY)
-		sprintbuf(pb, "\n");
+		printbuf_memappend(pb, "\n", 1);
 	for(ii=0; ii < json_object_array_length(jso); ii++)
 	{
 		struct json_object *val;
 		if (had_children)
 		{
-			sprintbuf(pb, ",");
+			printbuf_memappend(pb, ",", 1);
 			if (flags & JSON_C_TO_STRING_PRETTY)
-				sprintbuf(pb, "\n");
+				printbuf_memappend(pb, "\n", 1);
 		}
 		had_children = 1;
 		if (flags & JSON_C_TO_STRING_SPACED)
-			sprintbuf(pb, " ");
+			printbuf_memappend(pb, " ", 1);
 		indent(pb, level + 1, flags);
 		val = json_object_array_get_idx(jso, ii);
 		if(val == NULL)
-			sprintbuf(pb, "null");
+			printbuf_memappend(pb, "null", 4);
 		else
 			val->_to_json_string(val, pb, level+1, flags);
 	}
 	if (flags & JSON_C_TO_STRING_PRETTY)
 	{
 		if (had_children)
-			sprintbuf(pb, "\n");
+			printbuf_memappend(pb, "\n", 1);
 		indent(pb,level,flags);
 	}
 
 	if (flags & JSON_C_TO_STRING_SPACED)
-		return sprintbuf(pb, " ]");
+		return printbuf_memappend(pb, " ]", 2);
 	else
-		return sprintbuf(pb, "]");
+		return printbuf_memappend(pb, "]", 1);
 }
 
 static void json_object_array_entry_free(void *data)