utx::u16str_size - A function object to get the length of a u16 string: get the character counter.
The function object is the object of instantiated template class utx::str_size_class .
It gets string size by template type traits.
It gets string size by searching until '\0' is found.
It works on both string literal or string made by new.
For utx::str_size, type_xt is char; for utx::wstr_size, type_xt is wchar_t, ..., etc.