Support task executors

[gjcairo]

swift-async-algorithms spawns unstructured tasks in several places (like for the merge operation). This is problematic because it doesn’t propagate the current preferred executor to these tasks. This means that it’s impossible to compose usage of these operations with custom task executors.

This should be fixed, either by seeing if it's possible to refactor these operations to use structured concurrency, or by setting a task executor on these unstructured tasks.