Let AbsAlgAss no longer be a Ring

[lgoettgens]

Fixes #1139.

Let's see if anything breaks. If not, I see no reason not to change this.
However, I would classify this a breaking change.

[lgoettgens]

I am unsure what else from #441 needs to be reverted.

[thofma]

Lets wait for the tests

[lgoettgens]

AlgAssAbsOrd and AlgAssRelOrd then must be changed from <: Ring to <: NCRing as well, right? @thofma

[thofma]

Yes, correct

[lgoettgens]

This needs a few more changes in AbstractAlgebra (Nemocas/AbstractAlgebra.jl#1423). The relevant test files succeed locally (with Nemocas/AbstractAlgebra.jl#1423 and Nemocas/Nemo.jl#1531 deved in). I am currently running the full testsuite locally.

[lgoettgens]

I think this needs to be restarted once #1208 is merged.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.01% ⚠️

Comparison is base (1cf978a) 74.44% compared to head (ddc2491) 74.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1206      +/-   ##
==========================================
- Coverage   74.44%   74.43%   -0.01%     
==========================================
  Files         346      346              
  Lines      110747   110745       -2     
==========================================
- Hits        82442    82434       -8     
- Misses      28305    28311       +6     

Files Changed	Coverage Δ
src/AlgAssAbsOrd/Ideal.jl	74.91% <ø> (ø)
src/AlgAssAbsOrd/Order.jl	80.88% <ø> (ø)
src/AlgAssAbsOrd/Types.jl	89.10% <ø> (ø)
src/AlgAssRelOrd/Types.jl	86.79% <ø> (ø)
src/AlgAss/AlgMat.jl	87.75% <100.00%> (ø)
src/AlgAss/Types.jl	87.50% <100.00%> (+1.13%)	⬆️

... and 35 files with indirect coverage changes

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