Complete `_CoqProject` file with the list of the files for `coq_makefile`.
Update `.gitignore` with files produced by `coqdoc`.
Cosmetic improvements.

- Typos and english in comments
- Changed name of unbiaised finite union from `bind` to `Union` 
- Better `simpl` behaviour for set primitives (which are supposed to be reduced by lemmas, via the `set_simplify` hint base).