[#73] fix(lib): use jemallocator only on UNIX

[M0dEx]

This PR fixes the following issues:

• Building jemallocator on Windows seems like a hassle with the required dependencies and compilation errors, it will from now on be enabled on UNIX systems only.
• Terminal colours got broken in https://github.com/M0dEx/quincy/pull/63, now they should work again.

The PR also bumps tun2 to version ^2.0.0.

Closes #73.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 82.35%. Comparing base (a421a18) to head (bb14a9c).

Files	Patch %	Lines
src/utils/tracing.rs	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #74 +/- ## ======================================= Coverage 82.35% 82.35% ======================================= Files 16 16 Lines 918 918 ======================================= Hits 756 756 Misses 162 162 ```

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