Storing the list of all exponents in the same vector

[blegat]

It could be a performance win to change this Vector{Vector{Int}} into Vector{Int}: https://github.com/JuliaAlgebra/DynamicPolynomials.jl/blob/a0e6d3e00b9824c06d1263f88671fdbf023fff2d/src/monomial_vector.jl#L6 We could also use a Matrix{Int} but then adding exponents in the list might be more tricky.