Equality operator

[cameel]

We really need an equality operator. In the (tic tac toe demo](https://github.com/leonardoalt/SolPrologV2/tree/master/src/demo) I had to work around it with clauses like these:

same(., .).
same(x, x).
same(o, o).
same([], []).
same([H1|T1], [H2|T2]) :- same(H1, H2), same(T1, T2).

In addition to requiring more code, they're also very heavy to evaluate in the current implementation due to deep nesting. Replacing it with an operator that just does term comparison would speed up the demo a lot.