Nit: Singlethreaded executors also don't require "Send", since they don't move things between threads. That allows you too e.g. use non-atomically refcounted th

Nit: Singlethreaded executors also don't require "Send", since they don't move things between threads. That allows you too e.g. use non-atomically refcounted things (Rc<T>) on singlethreaded executors, which you can't use in the multithreaded versions.