Rework implementation to use `Fiber#transfer`. Fixes #23.

Ruby 3 allows mixing `Fiber#resume/#yield` and `Fiber#transfer`. We can
take advantage of that to minimise the impact of non-blocking operations
on user flow control.

Previously, non-blocking operations would invoke `Fiber.yield` and this
was a user-visible side-effect. We did take advantage of it, but it also
meant that integration of Async with existing usage of Fiber could be
problematic. We tracked the most obvious issues in `enumerator_spec.rb`.

Now, non-blocking operations transfer directly to the scheduler fiber
and thus don't impact other usage of resume/yield.