write a Legion scheduler

I think there is a good story we can tell about how stdexec can add value to Legion. Consider:

• We intend to write a Legion scheduler for stdexec
• We intent to write async versions of the Thrust algorithms that take schedulers

Putting 2 and 2 together, we can make it utterly trivial for anybody currently using Thrust (lots of developers) to target the Legion runtime:

// Execute a Thrust algorithm on Legion:
auto [results...] = sync_wait(thrust_algorithm(legion_scheduler, arguments...)).value();

That's obviously just scratching the surface.

Start by reading stdexec's scheduler model and the Legion and Thrust APIs mentioned in the issue. Define the scope for a Legion scheduler and async Thrust algorithms that accept schedulers, then validate the proposed sync_wait(thrust_algorithm(legion_scheduler, arguments...)) usage as the completion target.