feat: Add sink::from_fn

[Marwes]

Felt the need for this a couple times to mock a sink or inject something
that is sink-like, but doesn't implement sink itself.

[Nemo157]

See previously #1493, actually capturing state inside the closure is painful.

Comparing the implementations I see two bugs here:

• poll_ready needs to flush, otherwise it will never return Ready.
• poll_flush needs to clear the future, otherwise flushing again after it's flushed will panic.

[Marwes]

See previously #1493, actually capturing state inside the closure is painful.

A mutex does the trick when that is necessary 🤷 . There are also cases where there isn't a need to capture state (say pushing to an external database.

[taiki-e]

I have no objection to adding a way to convert impl Fn* -> impl Future to Sink, as sink doesn't currently have an API to do it.

However, I'm not clear if this implementation is preferred. We tried to add something similar to stream (#1609 #1842), but we decided that it would be preferable to provide only unfold due to the limitations of async closure.

So, I'm starting to feel that adding an API like unfold that passes the state separately from the closure is better. @Marwes @Nemo157 Any thoughts?

[Marwes]

#2268

[taiki-e]

Closing this in favor of #2268.