First stab at more functionality for Laurent polynomial rings

[thofma]

To get started with some ideal and map functionality for Laurent polynomial rings (multivariate).

The main application so far is to do the following:

julia> QQxy, (x, y) = LaurentPolynomialRing(QQ, [:x, :y]);

julia> I = ideal(QQxy, [1 + x^-1, y + 1]);

julia> R, (xx, yy) = polynomial_ring(QQ, ["xx", "yy"]);

julia> f = hom(R, QQxy, gens(QQxy));

julia> preimage(f, I)
ideal(yy + 1, xx + 1)

This at the moment a bit too slow for the application, but I probably did something not too clever. What we need to do: Given an ideal $I$ of $K[x_1^{\pm},\dotsc,x_n^{\pm}]$ determine $I \cap K[x_1,\dotsc,x_n]$. We can assume that $I = \langle f_1,\dotsc,f_n \rangle$ with $f_i \in K[x_1,\dotsc,x_n]$. At the moment we do

1. Let $\tilde I = \langle f_1,\dotsc, f_n \rangle \subseteq K[x_1,\dotsc,x_n]$ the ideal generated in the polynomial ring.
2. Saturate $\tilde I$ at $x_1,\dotsc,x_n$.

At the moment I am doing step 2. iteratively, but there is problem a better way? Happy to hear what the commutative algebra wizards have to say (@ederc @wdecker).

[codecov]

Codecov Report

Merging #2448 (ac636d3) into master (a94bc0e) will increase coverage by 1.40%.
The diff coverage is 84.82%.

❗ Current head ac636d3 differs from pull request most recent head 0f8e611. Consider uploading reports for the commit 0f8e611 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2448      +/-   ##
==========================================
+ Coverage   71.62%   73.03%   +1.40%     
==========================================
  Files         395      398       +3     
  Lines       53023    53189     +166     
==========================================
+ Hits        37979    38845     +866     
+ Misses      15044    14344     -700     

Impacted Files	Coverage Δ
experimental/Laurent/src/Laurent.jl	80.45% <80.45%> (ø)
experimental/Laurent/src/Types.jl	100.00% <100.00%> (ø)
experimental/Laurent/test/runtests.jl	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

[thofma]

I think this is a good start and can be merged. We will see how much it will benefit from the improvements to the saturation.

[fingolfin]

Looks good to me but of course @wdecker should ideally also have a look...? Or we just merge and let him complain later ;-)