fix: handle index expression on binary string type

_test/str3.go

@@ -0,0 +1,11 @@
+package main
+
+import "strconv"
+
+func main() {
+	str := strconv.Itoa(101)
+	println(str[0] == '1')
+}
+
+// Output:
+// true

interp/cfg.go

@@ -634,7 +634,11 @@ func (interp *Interpreter) cfg(root *node) ([]*node, error) {
 			case stringT:
 				n.typ = sc.getType("byte")
 			case valueT:
-				n.typ = &itype{cat: valueT, rtype: t.rtype.Elem()}
+				if t.rtype.Kind() == reflect.String {
+					n.typ = sc.getType("byte")
+				} else {
+					n.typ = &itype{cat: valueT, rtype: t.rtype.Elem()}
+				}
 			default:
 				n.typ = t.val
 			}