Scope overhaul

[NthTensor]

This is a large-scale overhaul of the scope implementation.

• Loosen an overly restrictive memory ordering for the scope counter.
• Switch from rayon-style to bevy-style scopes (two lifetimes instead of one). This allows references to scopes to be passed around more freely, and will reduce migration pain down the road.
• Make scopes properly panic safe by porting over the panic propagation from rayon.
• Pass the worker to blocking tasks spawned on scopes, just like we do for static tasks.
• Clean up documentation and generally spruce things up a bit.
• Remove pointless use of pin in favor of an unsafe assertion that scopes don't move. Pin turned out to be far more haste than it was worth.
• Remove the ability to spawn async functions on scopes. I'll be removing this from thread pool soon as well. Futures are simpler to work with, and this created some nontrivial duplication for little benefit.

[hymm]

just saw that you merged this. Code looks good, just had a couple non-blocking comments.

[hymm]

you might want to add a test for nested calls to scope. That was something that I saw deadlock in a couple of the other executors that I tested. In this case it'd be something like

        THREAD_POOL.with_worker(|worker| {
            scope(|scope| {
                scope.spawn_on(worker, |_| {
                    THREAD_POOL.with_worker(|worker| {
                         scope(|scope| {
                             scope.spawn_on(worker, |_| {
                                 string = "b";
                             })
                        });
                    });
                })
            });
        });