perf: improve performance of Baker.get_fields()

[joekohlsdorf]

Describe the change
#352 was meant to be a cosmetic change which introduced a performance regression.

This PR reduces the time complexity of get_fields() from O(n*m) to O(n).

I have a project of significant size (over 50k tests), this change reduces the total runtime of the test suite by ~200% back to what we saw before release 1.8.0.
I also tried other approaches, for example using filter or turning this into a cached property but these didn't achieve a better result.

PR Checklist

• Change is covered with tests
• CHANGELOG.md is updated

[amureki]

@joekohlsdorf hey Joe, this is a fantastic suggestion! Many thanks for reporting the issue, finding the regression and fixing it altogether! 🥇
Happy to hear that model-bakery serves your project with ~50k tests, this is a good note to me to care about the performance here.

I am merging this and issuing a new release right away.