coroutine: experimental: generator: implement move and swap

[bhalevy]

The coroutine generator move ctor, move assignment operator, and swap override were never fully implemented.

This patch completes their implementation
and adds unit tests to cover move and swap
and also moving not-drained generator (to test
moving of their buffered value(s)).

Fixes #1789

[tchaikov]

@bhalevy hi Benny, thank you for addressing this issue. but i think the more fundamental problems of the generator are

• Abandoned generator leaks memory #2190
• the "Generator fallback test" in directory_test is flaky due to !_wait_for_free_space assertion failure #1913
• coroutine generator use-after-free #1677

I believe we should address these issues before fixing the move semantics of the generator. I agree that they are the next important matters to tackle. Perhaps you could help review pull request #2218? Once it's merged, we should be in a better position to resolve issue #1789. What are your thoughts on this approach?

[bhalevy]

@bhalevy hi Benny, thank you for addressing this issue. but i think the more fundamental problems of the generator are

• Abandoned generator leaks memory #2190
• the "Generator fallback test" in directory_test is flaky due to !_wait_for_free_space assertion failure #1913
• coroutine generator use-after-free #1677

I believe we should address these issues before fixing the move semantics of the generator. I agree that they are the next important matters to tackle. Perhaps you could help review pull request #2218? Once it's merged, we should be in a better position to resolve issue #1789. What are your thoughts on this approach?

I have no objection.
Just please add the additional tests in the PR to #2218 to thoroughly test move semantics of the async_generator.