Resolve "Optimisation of shift algorithm" ✨️

 🐔️👍️
Approved-by: belazy <aarthuur01@gmail.com>
Approved-by: loutr <loutr@crans.org>
Approved-by: v-lafeychine <vincent.lafeychine@proton.me>
Approved-by: aalbert <augustin.albert@bleu-azure.fr>

🦀️🍰🦀️🍰🦀️🍰

* fix(kernel::memory::term): fix incorrect initialisation for is_certainly_closed

* chore: edit documentation & comments

* feat(kernel): Finding certain terms closedness while building them

* fix(kernel): False positive closed term with offset == 0

* feat(kernel): Set 'is_certainly_closed' if effectively closed

* feat(kernel): Add closedness boolean to term header & jump shift if term is closed