Added IBinaryRelation<TLeft, TRight>

Interface for different binary relation implementations.
Revert "Added IBinaryRelation<TLeft, TRight>"

This reverts commit 5d7b099e389ea2268fd5471fc51f522878d385c9.
Added relation infra to opt.Domain

Very basic stuff, to start with.
opt.Domain Relations update

- Added XML comments;
- Renamed IBinaryRelationChecker to IBinaryRelationValidator;
- Added unit tests for RelationBase.
Added relations to opt.Core

- Two binary relation implementations (Inequality and Set);
- Partial unit tests;
- Validators.