Implement / for Rational

[blegat]

It is used
https://github.com/JuliaAlgebra/MultivariatePolynomials.jl/blob/c9cbca9fcc0d9d6bafced0d983d20e673a50a1bd/src/division.jl#L271
and it should be used
https://github.com/JuliaAlgebra/MultivariatePolynomials.jl/blob/c9cbca9fcc0d9d6bafced0d983d20e673a50a1bd/src/division.jl#L53
Once it's done, we should see nice improvements for JuliaAlgebra/MultivariatePolynomials.jl#194