Make ranges constexpr formattable

[ilazaric]

Added ranges-formatting FMT_COMPILE tests were not working previously. This is because of a discrepancy between compile.h and format.h, former was using std::back_inserter(std::string), while latter was using back_appender(basic_memory_buffer).

This is actually needed because a different part has a bug:

  template <typename FormatContext, typename F, typename... T>
  FMT_CONSTEXPR auto write(FormatContext& ctx, const F& f, T&&... values) const
      -> decltype(ctx.out()) {
    auto specs = specs_;
    handle_dynamic_spec(specs.dynamic_width(), specs.width, width_ref_, ctx);
    if (specs.width == 0) return f.write_body(ctx, static_cast<T&&>(values)...);

    auto buf = basic_memory_buffer<Char>();
    auto buffer_ctx =
        FormatContext(basic_appender<Char>(buf), ctx.args(), ctx.locale());
    f.write_body(buffer_ctx, static_cast<T&&>(values)...);
    return detail::write<Char>(
        ctx.out(), basic_string_view<Char>(buf.data(), buf.size()), specs);
  }

The snippet tries to construct a FormatContext using a different iterator type.
It didn't seem obvious how to fix it, as ctx.args() is also context specific.

This PR is a cleanup of #4956