Reduce some code duplication, set some parents

[fingolfin]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.19608% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.23%. Comparing base (ae8c64d) to head (6ea004c).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/FlintTypes.jl	90.19%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1887   +/-   ##
=======================================
  Coverage   87.23%   87.23%           
=======================================
  Files          97       97           
  Lines       35758    35669   -89     
=======================================
- Hits        31194    31117   -77     
+ Misses       4564     4552   -12     

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[lgoettgens]

Changes look good.
I was a bit confused, that the constructors of e.g. ZZModPolyRingElem don't set any parent. Maybe this can be tackled once #1889 is through with separating the flint from the nemo fields.

[fingolfin]

As you probably know, a bunch of these Nemo types are designed to not require a parent to be set, so one does not have to bother with creating one.

Come to think of, perhaps that's why the lines z.parent = a.parent were missing in the first place: if the input has no parent set, then this line will produce an error. So perhaps some if isdefined(...) need to be added..

[fingolfin]

Resolved merge conflict and added checks like this:

    if isdefined(a, :parent)
      z.parent = a.parent
    end