Improve xchar support for std::bitset formatter

Signed-off-by: Vladislav Shchapov <vladislav@shchapov.ru>

include/fmt/std.h

@@ -184,7 +184,8 @@ FMT_END_NAMESPACE
 FMT_BEGIN_NAMESPACE
 FMT_EXPORT
 template <std::size_t N, typename Char>
-struct formatter<std::bitset<N>, Char> : nested_formatter<string_view> {
+struct formatter<std::bitset<N>, Char>
+    : nested_formatter<basic_string_view<Char>, Char> {
  private:
   // Functor because C++11 doesn't support generic lambdas.
   struct writer {
@@ -204,7 +205,7 @@ struct formatter<std::bitset<N>, Char> : nested_formatter<string_view> {
   template <typename FormatContext>
   auto format(const std::bitset<N>& bs, FormatContext& ctx) const
       -> decltype(ctx.out()) {
-    return write_padded(ctx, writer{bs});
+    return this->write_padded(ctx, writer{bs});
   }
 };