Ensure that various thread pool and scheduler headers are not included transitively by public headers

[msimberg]

Headers like scheduling_loop.hpp or scheduled_thread_pool_impl.hpp should not be necessary for users to include, even transitively, I think. We should check if they're not necessary, and make sure that they're not included accidentally by public API headers. We also explicitly instantiate schedulers/thread pools, so at least some headers should be possible to exclude.