refactor: Rename `out` parameters to `out_$something`.

[iphydf]

In preparation for supporting out as a type qualifier in parameters.

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This change is 

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 97.95918% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 73.14%. Comparing base (0199c0f) to head (3e05824).

Files	Patch %	Lines
toxcore/group_pack.c	71.42%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2726 +/- ## ========================================== + Coverage 72.99% 73.14% +0.14% ========================================== Files 149 149 Lines 30516 30516 ========================================== + Hits 22276 22321 +45 + Misses 8240 8195 -45 ```

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