[overlay][relate][is_valid][buffer] Remove other_id from turn_operation.
[overlay][is_valid] Replace int with signed_index_type for segments indexes.
[overlay] Move signed_index_type to separate file. Clean headers in ring_ and segment_identifier.hpp.
[test][setops] Fix invalid namespace in test_get_turns_ll_invariance.
Merge pull request #153 from awulkiew/feature/refactor_turns

Overlay/Turns refactorization