Have Equivalences::minimize take Option<Slice>

[frankmcsherry]

The minimize function for Equivalences took an &Option<Vec<T>>, which made sense in several initial call sites, but has led to the need to clone various things just to get an option around a vector. This PR changes the argument to be Option<&[T]>, which one can arrive at without cloning. The downside is a bit more chatter converting the optional vector to an optional slice, but .. just better practice anyhow.

Motivation

Tips for reviewer

Checklist

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