Propositon: Number, 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.

The Addable class resolve conflicts on - and + with Discrete.

To do next: 
* Use in the math module
* Support arithmetic operations between any numbers, maybe with double dispatch? This would ease the mix of Float and Int instances while still folowing co-variance rules. Or does the performance cost would be too high?
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class Int
    fun *(o: Number): OTHER
    ...
end
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