feat: more generic is_trivial

[thofma]

As suggested by @YueRen. All rings implement one and is_zero, but many don't or cannot implement characteristic.

Will make #1857 less breaking.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.17%. Comparing base (9d38ede) to head (82022c2).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1905   +/-   ##
=======================================
  Coverage   88.16%   88.17%           
=======================================
  Files         120      120           
  Lines       30296    30296           
=======================================
+ Hits        26711    26712    +1     
+ Misses       3585     3584    -1     

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[fingolfin]

So how about also extending the default characteristic method to have this test? I.e. it could return 1 if one(R) == zero(R)

[thofma]

I think there is no default characteristic method. Someone could add one later, but I would like to get this PR here merged first.