refactor: Use merge_sort instead of qsort for sorting.

[iphydf]

This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 98.68421% with 2 lines in your changes missing coverage. Please review.

Project coverage is 73.16%. Comparing base (a3d1b85) to head (6970387).
Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
toxcore/DHT.c	97.36%	1 Missing ⚠️
toxcore/group.c	95.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2638      +/-   ##
==========================================
+ Coverage   73.04%   73.16%   +0.11%     
==========================================
  Files         149      149              
  Lines       30516    30594      +78     
==========================================
+ Hits        22290    22383      +93     
+ Misses       8226     8211      -15     

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[JFreegman]

What are the advantages of merge sort over qsort in this instance?

[iphydf]

What are the advantages of merge sort over qsort in this instance?

https://github.com/TokTok/c-toxcore/pull/2638/files#diff-421ac668286fc646ef81a78b41a2b05bb4c1dbf1f02f3ed69ced2929a8ba4e10R141-R145

I guess I can explain more in that comment if needed. This PR is part of my attempt to make toxcore C code rely less on struct layouts and low level system properties and be semantically simpler. I know C coders like to bit-twiddle everything, but I think it's worth trying to avoid that as much as possible. qsort operates with memcpy, lots of casting from and to char*, moving around in abstract memory with pointer arithmetic, etc. It's semantically complex and requires complicated abstract domains that understand memory layouts and their implications.

[iphydf]

As for why merge sort instead of any other algorithm: it's an easy one to implement and has deterministic time complexity. If we have a lot of mostly-sorted arrays, we should probably just sorted-insert elements rather than adding a bunch of unsorted elements and sorting afterwards.