support input_iterator_tag in params and segments

Functions such as
https://github.com/CPPAlliance/url/blob/3b6e2afafcd8733ec453c2595e1d434913eed335/include/boost/url/params.hpp#L174
could be implemented for `std::input_iterator_tag`.

Since we pass through the parameters twice, it would require us to use an extra heap allocation for a `std::string` with the params before calling `edit_params`. This should be documented.