Introduce a new mock type to tests streams and eventually
sinks. Only includes next() and wait() for now. Fixes #4106

Within `notify_rx`, looping while re-locking and re-reading from
`Shared.tail` as long as there are still available wakers causes a
quadratic slowdown as receivers which are looping receiving from the
channel are added. Instead of continually re-reading from the original
list, this commit modifies `notify_rx` to move the waiters into a
separate list immediately similar to how `Notify::notify_waiters` works,
using a new `WaitersList` struct modified after NotifyWaitersList.

Fixes #5923

This patch aims to reduce the number of threads that get no-op wakeups.

Add `Ready::ERROR` enabling callers to specify interest in error readiness. Some platforms use error
readiness notifications to notify of other events. For example, Linux uses error to notify receipt of
messages on a UDP socket's error queue.

Using error readiness is platform specific.

Closes #5716

Closes #5777