Add proofs for Sequence methods

[Baptouuuu]

Recents bugs have shown that dealing with deferred sequences can lead to bugs due to internal state on iterators (#37). The behaviour is not the same as the ones with memoized or lazy sequences.

Traditional tests are not enough to make sure the behaviours are the same.

It would be interesting to find proofs applicable to the 3 implementations.

Doing this on sequences is enough as other structures such as Set are based on them.