Move stats into expr

[gatesn]

This is the next step towards inverting the vortex-expr -> vortex-array dependency. We need it to be the other way around.

Breaking Change

vortex::stats::Stat and other related types have moved to vortex::expr::stats::Stat

[codecov]

Codecov Report

❌ Patch coverage is 89.70588% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.98%. Comparing base (682289f) to head (3617fba).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
vortex-array/src/expr/stats/mod.rs	89.70%	7 Missing ⚠️

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