More results from the Spectral repository are moved to this library
Also make various type-class arguments of truncatedness and equivalences which were hard to synthesize explicit
Add more theorems about mapping cylinders, fibers, truncated 2-quotient, truncated univalence, pre/postcomposition with an iso in a precategory.
renamings: equiv.refl -> equiv.rfl and equiv_eq <-> equiv_eq'