Provide a non-Send bound spawn method and Task implementation.

[async-rs]

Provide a non-Send bound spawn method and Task implementation.

[kabergstrom]

FWIW I also forked async-task, though I actually removed all cross-thread synchronization (atomics) and related Send bounds.
https://github.com/kabergstrom/async-task/
Just did it to see how fast one could make an executor that doesn't support cross-thread synchronization in the Task object, instead using a lockless ring buffer per executor to receive tasks. These single-threaded executors can be composed into groups, where task spawning can then be spread over the executors with user-provided load balancing algorithms (round robin provided). It's 2-10x faster on my machine in the microbenches people were arguing about recently.
https://github.com/kabergstrom/lonely