core: private::weak_task::weaken_task: Take a closure that takes a PortOne instead of a Port

[catamorphism]

Comment says "I've experienced bugginess with oneshot pipes and try_send." If so, the bugs should be filed.

[bstrie]

The only users of weaken_task are the test cases and the old libuv support. It's possible that this function is obsoleted by the new runtime work.

[brson]

I do hope to remove this feature after we remove the old uv code.

[brson]

weak_task is dead.