some of the changes are backported from the hott3 library
pi_pathover and pi_pathover' are interchanged (same for variants and for sigma)
various definitions received explicit arguments: pinverse and eq_equiv_homotopy and ***.sigma_char
eq_of_fn_eq_fn is renamed to inj
in definitions about higher loop spaces and homotopy groups, the natural number arguments are now consistently before the type arguments
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'
