Add copy(::UnivPoly)

[fingolfin]

No description provided.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.92%. Comparing base (f69ac97) to head (bf0405d).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2178   +/-   ##
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  Coverage   87.92%   87.92%           
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  Files         127      127           
  Lines       31780    31783    +3     
=======================================
+ Hits        27943    27946    +3     
  Misses       3837     3837           

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[joschmitt]

I can't say that I ever used the universal polynomial ring, but this looks fine to me.

I am a bit wary that the copy of an MPoly might now share coefficients with the original polynomial as opposed to what it did before. But the tests pass, so we are good?!

[fingolfin]

You make a fair point, @joschmitt . I'll split this PR into two: one which adds the copy method for UnivPoly, and one with the "optimization" where we need to decide if that's what we want or not (perhaps not)