fix: change the behavior of printf in string op.

- add compiler hint for checking the arguments of printf.
- change the return value of printf. from std::expected to normal value. use C++ exception to indicate error.
	* the error of printf usually caused by programmer. so it can be found when testing program.
	* so i use std::logic_error to indicate this and programmer should fix this before releasing program.
- change the use of encoding convertion. for those cases that convertion must be safe, we unwrap it directly.

testbench/yycc/string/op.cpp

@@ -10,8 +10,16 @@ using namespace std::literals::string_view_literals;
 namespace yycctest::string::op {
 
     TEST(StringOp, Printf) {
-        auto rv = OP::printf(u8"%s == %s", u8"Hello World", u8"Hello, world");
-        EXPECT_EQ(rv, u8"Hello World == Hello, world");
+        // UTF8 string
+        {
+            auto rv = OP::printf(u8"%s == %s", u8"Hello World", u8"Hello, world");
+            EXPECT_EQ(rv, u8"Hello World == Hello, world");
+        }
+        // Ordinary string
+        {
+            auto rv = OP::printf("%s == %s", "Hello World", "Hello, world");
+            EXPECT_EQ(rv, "Hello World == Hello, world");
+        }
     }
 
     TEST(StringOp, Replace) {