Move 'priors' out of 'utils' and turn them into a separate module

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 98.07692% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 95.25%. Comparing base (64bbec2) to head (cb28ab8).

Files	Patch %	Lines
gpax/utils/fn.py	97.95%	1 Missing :warning:
tests/test_func_setter.py	97.82%	1 Missing :warning:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #94 +/- ## ========================================== - Coverage 95.26% 95.25% -0.02% ========================================== Files 55 58 +3 Lines 4269 4280 +11 ========================================== + Hits 4067 4077 +10 - Misses 202 203 +1 ``` | [Flag](https://app.codecov.io/gh/ziatdinovmax/gpax/pull/94/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Maxim+Ziatdinov) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/ziatdinovmax/gpax/pull/94/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Maxim+Ziatdinov) | `95.25% <98.07%> (-0.02%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Maxim+Ziatdinov#carryforward-flags-in-the-pull-request-comment) to find out more.

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