Deduplicate FLINT finalizers

[lgoettgens]

[...] several high-level Julia structs wrap the same underlying FLINT structs (e.g. fmpz_poly_struct is wrapped by ZZPolyRingElem, ZZAbsPowerSeriesRingElem and ZZRelPowerSeriesRingElem) and so they could use the same finalizer (well technically they already could do that now).
excerpt from #1889 (comment) by @fingolfin

implements exactly this by first moving all of them to a common block, renaming them w.r.t. the called flint function, sorting alphabetically, and then merging neighboring ones where appropriate.

[codecov]

Codecov Report

Attention: Patch coverage is 96.46465% with 7 lines in your changes missing coverage. Please review.

Project coverage is 88.29%. Comparing base (0644dcc) to head (4179f87).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
src/flint/FlintTypes.jl	96.44%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2008      +/-   ##
==========================================
- Coverage   88.31%   88.29%   -0.03%     
==========================================
  Files          98       98              
  Lines       36162    36101      -61     
==========================================
- Hits        31938    31876      -62     
- Misses       4224     4225       +1     

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