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
