Avoid base_ring for universal polynomial rings in ActionPolyRing

[SoongNoonien]

In my mission to completing Nemocas/AbstractAlgebra.jl#2182 I discovered that the experimental ActionPolyRing uses the universal polynomial rings. With this AbstractAlgebra PR the meaning of base_ring for universal polynomial rings will change and coefficient_ring should be used instead. Perhaps coefficient_ring should be implemented for the ActionPolyRing as well? (See: #1505 and Nemocas/AbstractAlgebra.jl#1207)

@SirToby25 I'm pinging you as you are mentioned in the docs of the ActionPolyRing.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.99%. Comparing base (b2f5345) to head (069ac33).
⚠️ Report is 14 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5427      +/-   ##
==========================================
- Coverage   83.99%   83.99%   -0.01%     
==========================================
  Files         721      721              
  Lines       98479    98479              
==========================================
- Hits        82718    82716       -2     
- Misses      15761    15763       +2     

Files with missing lines	Coverage Δ
experimental/ActionPolyRing/src/Content.jl	97.99% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[lgoettgens]

Perhaps coefficient_ring should be implemented for the ActionPolyRing as well?

I would agree with you. @SirToby25 could you take care of this?

[SirToby25]

Yes, I will do that. I will also change the functionality of base_ring in accordance with Nemocas/AbstractAlgebra.jl#2182, so that it replaces __upr

[lgoettgens]

Yes, I will do that. I will also change the functionality of base_ring in accordance with Nemocas/AbstractAlgebra.jl#2182, so that it replaces __upr

Actually, you cannot do that right now as this will need a breaking AA release first.
Thus, it would be best to split your work into two parts: The first one can be merged right now and just adds coefficient_ring. The second one can be merged right after the AA release (this can then replace the __upr thing).