Update fmt to 8.0.0; drop support for wchar

Adds some boost-nowide headers to assist in
UTF-8/16 conversion for Windows API.

include/nowide/detail/is_string_container.hpp

@@ -0,0 +1,94 @@
+//
+//  Copyright (c) 2020 Alexander Grund
+//
+//  Distributed under the Boost Software License, Version 1.0. (See
+//  accompanying file LICENSE or copy at
+//  http://www.boost.org/LICENSE_1_0.txt)
+//
+#ifndef NOWIDE_DETAIL_IS_STRING_CONTAINER_HPP_INCLUDED
+#define NOWIDE_DETAIL_IS_STRING_CONTAINER_HPP_INCLUDED
+
+#include <cstddef>
+#include <type_traits>
+
+namespace nowide {
+    namespace detail {
+        template<class...>
+        struct make_void
+        {
+            typedef void type;
+        };
+
+        template<class... Ts>
+        using void_t = typename make_void<Ts...>::type;
+
+        template<typename T>
+        struct is_char_type : std::false_type
+        {};
+        template<>
+        struct is_char_type<char> : std::true_type
+        {};
+        template<>
+        struct is_char_type<wchar_t> : std::true_type
+        {};
+        template<>
+        struct is_char_type<char16_t> : std::true_type
+        {};
+        template<>
+        struct is_char_type<char32_t> : std::true_type
+        {};
+#ifdef __cpp_char8_t
+        template<>
+        struct is_char_type<char8_t> : std::true_type
+        {};
+#endif
+
+        template<typename T>
+        struct is_c_string : std::false_type
+        {};
+        template<typename T>
+        struct is_c_string<const T*> : is_char_type<T>
+        {};
+
+        template<typename T>
+        using const_data_result = decltype(std::declval<const T>().data());
+        /// Return the size of the char type returned by the data() member function
+        template<typename T>
+        using get_data_width =
+          std::integral_constant<std::size_t, sizeof(typename std::remove_pointer<const_data_result<T>>::type)>;
+        template<typename T>
+        using size_result = decltype(std::declval<T>().size());
+        /// Return true if the data() member function returns a pointer to a type of size 1
+        template<typename T>
+        using has_narrow_data = std::integral_constant<bool, (get_data_width<T>::value == 1)>;
+
+        /// Return true if T is a string container, e.g. std::basic_string, std::basic_string_view
+        /// Requires a static value `npos`, a member function `size()` returning an integral,
+        /// and a member function `data()` returning a C string
+        template<typename T, bool isNarrow, typename = void>
+        struct is_string_container : std::false_type
+        {};
+        // clang-format off
+        template<typename T, bool isNarrow>
+        struct is_string_container<T, isNarrow, void_t<decltype(T::npos), size_result<T>, const_data_result<T>>>
+            : std::integral_constant<bool,
+                                     std::is_integral<decltype(T::npos)>::value
+                                       && std::is_integral<size_result<T>>::value
+                                       && is_c_string<const_data_result<T>>::value
+                                       && isNarrow == has_narrow_data<T>::value>
+        {};
+        // clang-format on
+        template<typename T>
+        using requires_narrow_string_container = typename std::enable_if<is_string_container<T, true>::value>::type;
+        template<typename T>
+        using requires_wide_string_container = typename std::enable_if<is_string_container<T, false>::value>::type;
+
+        template<typename T>
+        using requires_narrow_char = typename std::enable_if<sizeof(T) == 1 && is_char_type<T>::value>::type;
+        template<typename T>
+        using requires_wide_char = typename std::enable_if<(sizeof(T) > 1) && is_char_type<T>::value>::type;
+
+    } // namespace detail
+} // namespace nowide
+
+#endif