Add more setcoeff! methods and use them to deduplicate PolyRingElem constructors

[lgoettgens]

Unfortunately this won't work as I try to access attributes/parents of elements in setcoeff! before they are set.
I see two ways to resolve this issue:

1. All element constructors should get a parent. This will be a bigger change, in particular if we want to have this consistent for all poly types.
2. setcoeff! gets an optional fourth argument, that is a UInt for nmod_poly-based types and a fmpz_mod_ctx_struct for fmpz_mod_poly-based types. If this fourth argument is not passed, we can reconstruct it (in many cases, but not inside of the constructors as parents are not available).

I would tend to 2 for now, and maybe remove this fourth argument again once 1 has made enough progress. What do you think @fingolfin ?

[codecov]

Codecov Report

Attention: Patch coverage is 84.48276% with 18 lines in your changes missing coverage. Please review.

Project coverage is 87.88%. Comparing base (2876db2) to head (3f86479).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/fmpz_mod_abs_series.jl	33.33%	6 Missing ⚠️
src/flint/fmpz_mod_rel_series.jl	37.50%	5 Missing ⚠️
src/flint/nmod_abs_series.jl	66.66%	3 Missing ⚠️
src/flint/nmod_rel_series.jl	66.66%	3 Missing ⚠️
src/flint/fmpz_mod_poly.jl	90.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1911      +/-   ##
==========================================
- Coverage   87.89%   87.88%   -0.01%     
==========================================
  Files          99       99              
  Lines       36366    36181     -185     
==========================================
- Hits        31965    31799     -166     
+ Misses       4401     4382      -19     

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[lgoettgens]

This now implements option 2 of the initial message.