Fix .gitignore
* Fix a bug in the typechecking of eMH.
* Fix a bug in type externalization.
* Extend the specification with a new restriction: overloaded names
  and let-bound names live in two different spaces.

* sujet.pdf:
  - Alleviate the tasks.
  - Authorize the project to be done by a team of two students
    (but with extra tasks)
* Update sujet.pdf. A wrong version was pushed earlier.
* Fix typos.
* Front: Fix invalid overwriting of source files while processing .mle files.
* Minor clarification about the distinction between variables and methods names.
Merge remote-tracking branch 'upstream/master'
* Fix a bug in the OCaml pretty-printer.
* sujet.pdf: Minor remarks about identifiers.
* value_definition: Fix an invalid threading of the typing environment .
Merge remote-tracking branch 'upstream/master'