refactor: enforce immutability

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 90.90909% with 4 lines in your changes missing coverage. Please review.

Project coverage is 94.55%. Comparing base (c732bd2) to head (0ef2797).

Files with missing lines	Patch %	Lines
src/lib.rs	90.90%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #4 +/- ## ========================================== + Coverage 93.43% 94.55% +1.12% ========================================== Files 1 1 Lines 198 202 +4 ========================================== + Hits 185 191 +6 + Misses 13 11 -2 ```

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[larseggert]

Might also want to do this for the Windows addr_table and if_table pointers?

[mxinden]

I updated the windows implementation to use early returns like the unix implementation does. Let me know if you prefer the immutability change only.