Existing Class now works with universe polymorphism. Fixes HoTT bug #063
Safer entry point of primitive projections in the kernel, now it does recognize
a projection constant only of the form
 λ params (r : I params), match r with BuildR args => args_i end, without any
other user input and relies on it being typable (no more primitive projections
escaping this).
Fix a de Bruijn bug in checking code of projections.
- Fix the de Bruijn problem in check_projection for good :)
- Fix HoTT coq bug #80, implicit arguments with primitive projections were
wrongly automatically infered.
Fix type inference of the record argument of a projection to catch ill-typed
applications earlier.