Generic.Ideal: add missing deepcopy; add and use check_base_ring helper

[fingolfin]

Also reformat some code.

Extracted from PR #2108

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 88.00%. Comparing base (2627b5e) to head (d0825c5).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
src/generic/SparsePoly.jl	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2236   +/-   ##
=======================================
  Coverage   88.00%   88.00%           
=======================================
  Files         127      127           
  Lines       31798    31809   +11     
=======================================
+ Hits        27983    27995   +12     
+ Misses       3815     3814    -1     

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[SoongNoonien]

The changes seem fine. But I think we should add an explicit test which verifies that deepcopy does what it should when intersecting ideals.

[fingolfin]

I've added the check_base_ring from PR #2108 and inserted it into various Generic.Ideal method. That triggers the new deepcopy method -- and without it, the tests would fail:

Generic.Ideal.adhoc_multiplication |  396    396  0.4s
Generic.Ideal.intersect: Error During Test at /Users/mhorn/Projekte/OSCAR/AbstractAlgebra.jl/test/generic/Ideal-test.jl:667
  Test threw exception
  Expression: is_subset(K, I)
  base rings do not match
  Stacktrace:
   [1] error(s::String)
     @ Base ./error.jl:44
   [2] check_base_ring
     @ .../AbstractAlgebra.jl/src/AbstractAlgebra.jl:238 [inlined]
   [3] check_base_ring
     @ .../AbstractAlgebra.jl/src/AbstractAlgebra.jl:237 [inlined]
   [4] issubset(I::AbstractAlgebra.Generic.Ideal{AbstractAlgebra.Generic.Poly{AbstractAlgebra.GFElem{Int64}}}, J::AbstractAlgebra.Generic.Ideal{AbstractAlgebra.Generic.Poly{AbstractAlgebra.GFElem{Int64}}})
     @ AbstractAlgebra.Generic .../AbstractAlgebra.jl/src/generic/Ideal.jl:2122
...