refactor: finish Windows encoding namespace.

- finish Windows encoding namespace.
- add std::expected polyfill for help.

src/yycc/patch/expected.hpp

@@ -0,0 +1,40 @@
+#pragma once
+#include <type_traits>
+#include <variant>
+
+/**
+ * @brief A stupid polyfill for std::expected,
+ *
+ * For those C++ standard which do not support std::expected,
+ * we provide this namespace with a pretty bad but at least working std::expected pplyfill.
+ *
+ * The polyfill was done by std::variant.
+ *
+ * This namespace is used by this project because this framework must support C++ 17.
+ */
+namespace yycc::patch::expected {
+
+    template<typename TValue, typename TError, std::enable_if_t<!std::is_same_v<TValue, TError>, int> = 0>
+    using Expected = std::variant<TValue, TError>;
+
+    template<typename TValue, typename TError>
+    bool is_value(const Expected<TValue, TError>& exp) {
+        return exp.index() == 0u;
+    }
+
+    template<typename TValue, typename TError>
+    bool is_error(const Expected<TValue, TError>& exp) {
+        return exp.index() == 1u;
+    }
+
+    template<typename TValue, typename TError>
+    const TValue& get_value(const Expected<TValue, TError>& exp) {
+        return std::get<0>(exp);
+    }
+
+    template<typename TValue, typename TError>
+    const TError& get_error(const Expected<TValue, TError>& exp) {
+        return std::get<1>(exp);
+    }
+
+}