Change tree representation in Mini_calc to use different environments.
Update the documentation.
Perform Pierre's optimisation proposition in the code engine, adapt the parser and the interface to
match.

 - Replace strings by ints
 - Replace list by arrays
Little cheat to draw 1px lines with cairo.
Change completly the parser and the syntax, move to menhir and adapt bank_lsystem accordingly.
Ignore Emacs' temp files