When using LocalRuntime, it's possible to guarantee that certain callbacks on the runtime builder only run on the current thread. So we should provide non-Send versions of those callbacks.

Note that we should have tests for this. For example, it's very important that on_thread_park only runs inside Runtime::block_on calls. If it also runs in Handle::block_on calls, then this logic does not work. There must be tests for that before non-Send versions of the callbacks can be accepted.