Fix and use setcoeff! for arb poly rings

[lgoettgens]

The former dispatch for ZZRingElem used a non-existing flint function. I rewrote the setcoeff!s using flintify to dispatch to all possible flint functions.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 87.46%. Comparing base (06ec014) to head (6775ff3).
Report is 10 commits behind head on master.

Files with missing lines	Patch %	Lines
src/arb/ComplexPoly.jl	57.14%	3 Missing ⚠️
src/arb/RealPoly.jl	57.14%	3 Missing ⚠️
src/arb/acb_poly.jl	57.14%	3 Missing ⚠️
src/arb/arb_poly.jl	57.14%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1916      +/-   ##
==========================================
+ Coverage   87.39%   87.46%   +0.06%     
==========================================
  Files          97       97              
  Lines       35614    35626      +12     
==========================================
+ Hits        31124    31159      +35     
+ Misses       4490     4467      -23     

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[fingolfin]

Unfortunately this has conflicts now :-(