Restyle refactor: Use merge_sort instead of qsort for sorting.

[restyled-io]

A duplicate of #2638 with additional commits that automatically address
incorrect style, created by Restyled.

Since the original Pull Request was opened as a fork in a contributor's
repository, we are unable to create a Pull Request branching from it with only
the style fixes.

The following Restylers made fixes:

• clang-format

To incorporate these changes, you can either:

1. Merge this Pull Request instead of the original, or

2. Ask your contributor to locally incorporate these commits and push them to
   the original Pull Request

   Expand for example instructions

   ```console
   git remote add upstream https://github.com/TokTok/c-toxcore.git
   git fetch upstream pull/<this PR number>/head
   git merge --ff-only FETCH_HEAD
   git push
   ```
   

NOTE: As work continues on the original Pull Request, this process will
re-run and update (force-push) this Pull Request with updated style fixes as
necessary. If the style is fixed manually at any point (i.e. this process finds
no fixes to make), this Pull Request will be closed automatically.

Sorry if this was unexpected. To disable it, see our documentation.

---

This change is 

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (8b05296) 73.65% compared to head (88cd6a2) 73.68%.

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

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2639      +/-   ##
==========================================
+ Coverage   73.65%   73.68%   +0.02%     
==========================================
  Files         148      148              
  Lines       30419    30497      +78     
==========================================
+ Hits        22406    22471      +65     
- Misses       8013     8026      +13     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.