Review our use of impl Stream in e.g. Match queries

[alexjpwalker]

Currently our Match queries return impl Stream<Item = ConceptMap>.

This becomes a particular problem in the Concept API. We want to define methods such as get_instances in a trait, but trait methods cannot return impl Stream.

It might be more useful to users if they knew the exact type of Stream. Trouble is, that type is a super-complex type.

We should investigate the performance impact of using BoxStream. If not, we may need to not define these methods in traits. It may be worth consulting the Rust API guidelines.

[flyingsilverfin]

This is expected and we no longer use this API