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>