IO thread-pool and device

[angeloskath]

At this point this PR is more of a cautionary tale and a lesson for the future, I don't think this should ever be merged.

The interesting bits:

• This implements a thread pool that synchronizes within itself and could be used for interop parallelism on the CPU.
• Refactors a tiny bit the cpu synchronization task and computation task similar to metal which I think looks nicer.

The ugly bits:

• Most IO happens on a few files (often just one) which needs to be serialized with a mutex anyway.
• Dealing with synchronization between threads is actually complicated and we can't rely on array events as they can be out of order (and they aren't even signaled for within a stream). The solution is not the ugliest but it does use a lot of locks that can slow things down in latency critical situations (especially if there is contention).

[angeloskath]

I will just close this but keep the branch around in case we want to extract the cpu::make_task refactoring later.