`CombinationsSequence` should be a `Collection`

It's multipass (like almost every other `Sequence` in practice). Not being a collection limits its applicability in silly ways.

Start by locating CombinationsSequence and its existing Sequence conformance in the swift-algorithms repository, then inspect nearby sequence and collection implementations for the expected pattern. Confirm the collection requirements and add or update relevant tests so multipass behavior and collection operations are covered.

Written by the indexing model from the issue text.