Gmoshkin/fiber tweaks & more

Summary

• feat: Error::variant_name

• feat: fiber::JoinHandle::detach_and_forget & fiber::exists

• fix: deprecate the old fiber::Fiber api, use the new one everywhere in the code base

• feat: fiber::r#async::timeout::{deadline, IntoTimeout::deadline}

• feat: fiber::Cond::wait_deadline

• feat: fiber::JoinHandle::cancel, fiber::JoinHandle::wakeup

Ensure that

• New code is covered by tests
• API is documented
• Changelog is up to date
• Version is bumped in the appropriate Cargo.toml files