Optimise certain actions with predictable output to prevent errors due to cursors

[Baptouuuu]

One such action is calling Sequence::zip() with the same Sequence instance as source and argument. The result is the same as maping on the instance to produce tuples. But doing this on a deferred Sequence cause problems due to concurrently moving the cursor on the same object.

Other actions with predictable output are:

• equals
• intersect
• and probably more

An exhaustive check of all methods should be done to optimise these actions and prevent bugs.