Misc changes

[fingolfin]

• Remove redundant gcdinv methods
• Add isapprox methods for ZZRingElem & QQFieldElem
• Enhance fpPolyRingElem constructor to accept more Integer types
• Fix missing return values in two unsafe ops

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes missing coverage. Please review.

Project coverage is 87.23%. Comparing base (d457d82) to head (b7eb26a).

Files with missing lines	Patch %	Lines
src/flint/fmpq.jl	0.00%	4 Missing ⚠️
src/flint/fmpz.jl	0.00%	4 Missing ⚠️
src/HeckeMoreStuff.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1878      +/-   ##
==========================================
- Coverage   87.29%   87.23%   -0.06%     
==========================================
  Files          97       97              
  Lines       35745    35727      -18     
==========================================
- Hits        31202    31166      -36     
- Misses       4543     4561      +18     

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