Overlay/Turns refactorization

There are 2 goals for now:
1. To remove redundant `turn_operation<>::other_id` _(the first commit)_.
2. To replace `int` indexes in `segment_identifier` and various algorithms to `signed_index_type` equal to `std::ptrdiff_t` or `boost::make_signed<std::size_t>::type` _(in progress)_.

ad1. There are 2 places left where `other_id` can be found.
- `algorithms/detail/overlay/handle_tangencies.hpp`: in the commented debug code, I thought that it might be useful for someone, but I may remove it.
- `algorithms/detail/overlay/stream_info.hpp`: this looks like some debug tool for an old version of `turn_info` (when it contained data now in the `result_type` of `IntersectionPolicy`?), so AFAIU it shouldn't work. I wasn't sure if we should fix it or remove it.