Add set! for finite field elements

[lgoettgens]

ping @fingolfin to not work on the same thing

• fpField (skipping here due to other code structure)
• FpField (skipping here due to other code structure)
• fqPolyRepField (already done in More set! methods for fqPolyRepFieldElem #1810)
• FqPolyRepField / fq
• FqField / fq_default

[codecov]

Codecov Report

Attention: Patch coverage is 80.85106% with 9 lines in your changes missing coverage. Please review.

Project coverage is 87.20%. Comparing base (633112d) to head (c59c1f9).
Report is 15 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/fq.jl	66.66%	5 Missing ⚠️
src/flint/fq_default.jl	85.71%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1908      +/-   ##
==========================================
- Coverage   87.24%   87.20%   -0.04%     
==========================================
  Files          97       97              
  Lines       35590    35589       -1     
==========================================
- Hits        31049    31035      -14     
- Misses       4541     4554      +13     

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