Support kernel of maps with Laurent polynomials as codomain

[thofma]

@lkastner this allows:

julia> Hmat = matrix(ZZ, H)
[ 1   0]
[ 0   1]
[-2   5]
[-1   3]

julia> R,_ = polynomial_ring(QQ,nrows(Hmat));

julia> S,x = laurent_polynomial_ring(QQ, ncols(Hmat));

julia> f = hom(R, S, [prod(x.^Hmat[i,:]) for i in (1:nrows(Hmat))])
Ring homomorphism
  from multivariate polynomial ring in 4 variables over QQ
  to multivariate Laurent polynomial ring in 2 variables over QQ
defined by
  x1 -> x1
  x2 -> x2
  x3 -> x1^-2*x2^5
  x4 -> x1^-1*x2^3

julia> kernel(f)
Ideal generated by
  x2*x3 - x4^2
  -x1*x3 + x2^2*x4
  -x1*x4 + x2^3
  -x1*x3^2 + x2*x4^3
  -x1*x3^3 + x4^5

[lkastner]

Geil, danke!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 84.90%. Comparing base (b1aab33) to head (6b78415).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5078   +/-   ##
=======================================
  Coverage   84.90%   84.90%           
=======================================
  Files         706      706           
  Lines       95421    95424    +3     
=======================================
+ Hits        81017    81022    +5     
+ Misses      14404    14402    -2     

Files with missing lines	Coverage Δ
src/Rings/MPolyMap/AffineAlgebras.jl	96.69% <100.00%> (+0.08%)	⬆️

... and 1 file with indirect coverage changes

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