fix: ufmt multi-byte fix. (#1889)

<!-- please provide a detailed description of the changes made in this
pull request. -->

<details><summary>Contributors' checklist...</summary>

- [x] Added new tests, or not needed, or not feasible
- [ ] Provided an example (e.g. screenshot) to aid review or the PR is
self-explanatory
- [ ] Updated the official documentation or not needed
- [ ] No breaking changes were made, or a `BREAKING CHANGE: xxx` message
was included in the description
- [ ] Added references to related issues and PRs
- [ ] Provided any useful hints for running manual tests
- [ ] Added new benchmarks to [generated
graphs](https://gnoland.github.io/benchmarks), if any. More info
[here](https://github.com/gnolang/gno/blob/master/.benchmarks/README.md).
</details>

examples/gno.land/p/demo/ufmt/ufmt.gno

@@ -52,24 +52,26 @@ func Println(args ...interface{}) {
 //	%t: formats a boolean value to "true" or "false".
 //	%%: outputs a literal %. Does not consume an argument.
 func Sprintf(format string, args ...interface{}) string {
-	end := len(format)
+	// we use runes to handle multi-byte characters
+	sTor := []rune(format)
+	end := len(sTor)
 	argNum := 0
 	argLen := len(args)
 	buf := ""
 
 	for i := 0; i < end; {
 		isLast := i == end-1
-		c := format[i]
+		c := string(sTor[i])
 
-		if isLast || c != '%' {
+		if isLast || c != "%" {
 			// we don't check for invalid format like a one ending with "%"
 			buf += string(c)
 			i++
 			continue
 		}
 
-		verb := format[i+1]
-		if verb == '%' {
+		verb := string(sTor[i+1])
+		if verb == "%" {
 			buf += "%"
 			i += 2
 			continue
@@ -82,7 +84,7 @@ func Sprintf(format string, args ...interface{}) string {
 		argNum++
 
 		switch verb {
-		case 's':
+		case "s":
 			switch v := arg.(type) {
 			case interface{ String() string }:
 				buf += v.String()
@@ -91,7 +93,7 @@ func Sprintf(format string, args ...interface{}) string {
 			default:
 				buf += "(unhandled)"
 			}
-		case 'd':
+		case "d":
 			switch v := arg.(type) {
 			case int:
 				buf += strconv.Itoa(v)
@@ -116,7 +118,7 @@ func Sprintf(format string, args ...interface{}) string {
 			default:
 				buf += "(unhandled)"
 			}
-		case 't':
+		case "t":
 			switch v := arg.(type) {
 			case bool:
 				if v {

examples/gno.land/p/demo/ufmt/ufmt_test.gno

@@ -38,6 +38,9 @@ func TestSprintf(t *testing.T) {
 		{"no args", nil, "no args"},
 		{"finish with %", nil, "finish with %"},
 		{"stringer [%s]", []interface{}{stringer{}}, "stringer [I'm a stringer]"},
+		{"â", nil, "â"},
+		{"Hello, World! 😊", nil, "Hello, World! 😊"},
+		{"unicode formatting: %s", []interface{}{"😊"}, "unicode formatting: 😊"},
 	}
 
 	for _, tc := range cases {