This commit adds truncated 2-quotients, groupoid quotients, Eilenberg MacLane spaces, chain complexes, the long exact sequence of homotopy groups, the Freudenthal Suspension Theorem, Whitehead's principle, and the computation of homotopy groups of almost all spheres which are known in HoTT.
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'
This commit also removes many (now unnecessary) folds from the HoTT
library.
See issue #692
We still have to implement support for recursive definitions based on
brec_on that recurse over inductive families.
In hit.two_quotient we define a general construction to define hits with 2-dimensional path constructors, similar to quotients.
We can add 2-paths between any two 'words', where a word consists of 1-path constructors, concatenation and inverses.
We use this to define the torus, reflexive quotients and the reduced suspension.
There is still one 'sorry' in the construction
