Add implication: integral domains are not trivial

[fingolfin]

This way e.g. 'is_trivial(ZZ)' is compiled into 'reutrn false'.

If we also add a few more dedicated is_trivial methods for e.g. some quotient ring implementations, we may even be able to get by without caching this value anywhere?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.17%. Comparing base (77591f4) to head (360023c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1912      +/-   ##
==========================================
- Coverage   88.17%   88.17%   -0.01%     
==========================================
  Files         120      120              
  Lines       30298    30297       -1     
==========================================
- Hits        26716    26714       -2     
- Misses       3582     3583       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.