Add support for domain local await

[polytypic]

This PR adds domain local await support to Eio. Domain local await allows blocking abstractions to work across different schedulers.

I've also made a similar PR to Domainslib. If and when both Eio and Domainslib merge the support for domain local await, it should be possible to use upcoming version of kcas to easily communicate between Eio and Domainslib domains. Perhaps lockfree could also adopt use of domain local await until we get some official blocking mechanism.

[talex5]

Interesting. I guess this doesn't even need any changes in Eio. You could just run any Eio event loop and then wrap Domain_mgr to install this handler in all new domains, right?

[polytypic]

Interesting. I guess this doesn't even need any changes in Eio. You could just run any Eio event loop and then wrap Domain_mgr to install this handler in all new domains, right?

Yes, I think for many schedulers it would be possible to provide domain local await outside of the scheduler by implementing it in terms of the public API of the scheduler and then installing it within a fiber running on the scheduler. However, for better UX / DX it is preferable to have schedulers provide the support directly. Of course, DLA is experimental at this stage and it is quite possible that it might be replaced with some official standard blocking mechanism, but I think that for the short term it would be great to be able to have various libraries that need blocking to be able to cooperate through it.

[talex5]

Looks good (I just squashed and rebased it).