VCF adapter: missing `WhichPopulation` `impl`s for `Fn` and relatives

[laggycomputer]

While building MultiPopulationCounts, a T: WhichPopulation is required to map sample names to a population name. The simplest such T would be a simple function or closure, so it would be most ergonomic to implement WhichPopulation immediately for such a type. Because such an impl does not exist, and because of the orphan rule, it may be impossible to use a closure for WhichPopulation without a newtype.

[molpopgen]

Can you add some context to the body of this issue? What part of the API, etc., is this relevant to?