Function to express multivariate poly as univariate with coeffs in one less var

[wbhart]

We have something similar already implemented for the gcd function, but it is rather specialised.

It would be better to have

to_univariate(f, x)

return a polynomial that is a univariate in x (i.e. Poly) with coefficients in the same ring as f.

[wbhart]

Note that as this is a generic operation, it cannot be implemented in Flint. So the function will have to work for all polynomial types in AbstractAlgebra.MPolyElem, not just for Generic.MPoly's.

[nsajko]

This seems to be implemented now: https://nemocas.github.io/AbstractAlgebra.jl/dev/mpolynomial/#Conversions