* introduce hard limit to transactional * add single layer transactional * remove single_transactional * Update mod.rs * add tests * maybe fix contracts cc @athei * fmt * fix contract logic * Update frame/contracts/src/exec.rs Co-authored-by: Alexander Theißen <alex.theissen@me.com> * Update exec.rs * add unchecked and custom errors * Update lib.rs * Apply suggestions from code review Co-authored-by: Alexander Theißen <alex.theissen@me.com> * Replace storage access by atomics Co-authored-by: Alexander Theißen <alex.theissen@me.com>