feat: Fibers support

[domwst]

No description provided.

[rustbot]

Thank you for contributing to Miri! A reviewer will take a look at your PR, typically within a week or two.
Please remember to not force-push to the PR branch except when you need to rebase due to a conflict or when the reviewer asks you for it.

[domwst]

Sorry, ment to create a PR into my own repo for now

[RalfJung]

FWIW rust-lang/rust#148799 is adding code to Miri that relies on Fibers currently not being supported. Also please ensure to talk to us about motivation etc to make sure the feature is a good fit for Miri. Also see https://github.com/rust-lang/miri/blob/master/CONTRIBUTING.md#larger-scale-contributions.

[domwst]

Thank you for letting me know about conflicting feature and for contribution reference. Currently I'm just playing with the code to see how Miri could support fibers and what api makes sense. I'll surely discuss the design once I settle on a reasonable api/set of api's