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}:

DynamicPolynomials.jl/src/monomial_vector.jl

Line 6 in a0e6d3e

Z::Vector{Vector{Int}}

We could also use a Matrix{Int} but then adding exponents in the list might be more tricky.