Add is_trivial(::NCRing)

[fingolfin]

Also remove some outdated or pointless TODO comments, and
add a docstring for is_perfect

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 88.13%. Comparing base (5748ac4) to head (02b981c).
Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Rings.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1848      +/-   ##
==========================================
- Coverage   88.14%   88.13%   -0.01%     
==========================================
  Files         119      119              
  Lines       29978    29980       +2     
==========================================
  Hits        26423    26423              
- Misses       3555     3557       +2     

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see comment

[lgoettgens]

what about 0x0 matrix rings? they are trivial as well and lead to the same problems

[fingolfin]

I am tempted to say 0x0 matrix rings should just be not supported. For quotient rings we can't easily / cheaply check for them being trivial (as it may involve a Gröbner basis calculation or worse). But here it would be trivial.

For that matter, we definitely should prevent -1x-1 matrix rings 🤡

julia> matrix_ring(ZZ, -1)
Matrix ring of degree -1
  over integers

[fingolfin]

In any case, even if we decide to keep supporting 0x0 MatRing instances, the fix should be made to its characteristic function and seems out of scope for this PR?

[lgoettgens]

True, this PR delegates the question to characteristic. So this is good to go