Hello!

I found that if I put custom type as a named argument it is impossible to get its size in variadic functions:

template < typename Char >
void serialize(Buffer &buffer, const fmt::internal::NamedArg< Char >& arg) {
// serialize named argument
}

So I extend fmt::internal::NamedArg with no overhead:

template < typename Char, typename T >
struct NamedArgWithType : NamedArg< Char > {
NamedArgWithType(BasicStringRef< Char > argname, const T &value)
: NamedArg< Char >(argname, value) {}
};

With this fix I was able to cover all args serializaton/deserializaton for base and trivially copyable types.

Hope you'll accept this PR as well! Thanks in advance!