Merge: Numeric, a super-class to Int and Float

Will be used by my implementation of the Bentley-Ottmann algorithm,
allowing to be used on either Int or Float. Could also be used to
revamp the API of mnit::Display and to implement QuadTree.

Pull-Request: #344
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Julien Pagès <julien.m.pages@gmail.com

diff --cc lib/standard/kernel.nit
Simple merge

diff --cc src/abstract_compiler.nit
Simple merge

diff --cc src/naive_interpreter.nit
Simple merge