feat(kernel/type_checker): add `is_stuck` method, and improve ensure_pi method, closes #261
feat(library/normalize): add new flavors of normalize procedure
feat(library/unifier): improve FailLocal/FailCircular failures in the unifier by using normalization

This improvements was marked as TODO, and was preventing us from
elaborating the example in the new test vector3.lean
feat(library/unifier): improve FailLocal/FailCircular failures in the unifier by using normalization