chore(grit): don't use im::Vector where we don't have to

[arendjr]

Summary

Appears to give a few percent performance boost.

Test Plan

CI should remain green.

[codspeed-hq]

CodSpeed Performance Report

Merging #4203 will not alter performance

_{Comparing arendjr:replace-im-vector-with-vec (cf3d817) with main (bae50d1)}

Summary

✅ 105 untouched benchmarks