Missing `thread::scope()` API

The [`std::thread::scope()`](https://doc.rust-lang.org/std/thread/fn.scope.html) API from the standard library is very useful to create threads that capture their external environment (bound to a non-`'static` lifetime), and to auto-generate the thread-joining code. Unfortunately, it is missing from the [`loom::thread`](https://docs.rs/loom/latest/loom/thread/index.html) module.