Remove the 'static bound from everywhere

[alecmocatta]

This enables big performance improvements by not having to clone elements for ParallelStreams.

The complication is that, akin to crossbeam::scope, we need to block before returning to avoid the user mem::forgetting the handle and mutating the data while other threads still have a reference to it. If we were sync (like for example rayon) this wouldn't be much of an issue. As we're async and have to return whenever I/O blocks, this is harder to deal with.

The current compromise in this PR is using tokio to block the current task (when running under the multithreaded scheduler) or current thread (when running under the basic scheduler). This is unfortunately mildly surprising from a user POV. WIP.

TODO:

• wait for cancelled ThreadPool::spawned futures (e.g. during .all()) to ack that they've been cancelled, to avoid returning while a task may be running on another thread that uses referenced data
  • sync block - necessary for safety
  • async - optimisation to avoid blocking

See also:
tokio-rs/tokio#1879 tokio-rs/tokio#2576 tokio-rs/tokio#2596 rust-lang/rfcs#2958 rmanoka/async-scoped

There are two issues:

• It's possible to forget a borrow without running an associated destructor RFC: Alter mem::forget to be safe rust-lang/rfcs#1066. Result: we can't async yield to the user until tasks have been cancelled.
• Cancellation of tasks on other threads is async. Result: in JoinHandle::drop we must block on this async cancellation. Prefer some sort of async drop as an optimisation to avoid the blocking.

And a third:

• Cancellation points. Almost no code has them. What to do about that‽