lib: intro the `standard::numeric` module

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
calculator: move to a project folder

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
calculator: major logic refactor, move up display code & use numerics

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
calculator: add Makefile

Signed-off-by: Alexis Laferrière <alexis.laf@xymus.net>
Merge: Calculator refactor & numerics proposition

* Modularize calculator.nit into 3 modules and move to a project structure.
* Rewrite the code behind the displayed string.
* Use numerics to switch nicely between `Float` and `Int`
* No more weird bug with the C button.

The Android UI will follow in another PR.

Pull-Request: #646
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>