Add Stream wrappers in tokio-stream

[Darksonn]

This PR builds on #3342. When that PR is merged, the diff should become more easily readable.

In this PR I add wrappers for every Tokio type that previously implemented Stream and has an exposed poll method. Notably, the poll method is missing on Signal, so that type is not included in this PR.

[taiki-e]

The implementation LGTM.

However, I'm not sure if it's better to put everything in a single module (wrappers) or in multiple modules per feature (net, io, fs). What do you think?

[Darksonn]

I think the main place where we risk name overlaps is with receivers of various types, but those are all in sync.

[ipetkov]

Notably, the poll method is missing on Signal, so that type is not included in this PR.

@Darksonn the Signal types (both for Unix and Windows) have poll_recv methods which have the exact signature of Stream::poll_next, so we should be able to delegate the Stream impl to them

[Darksonn]

Yes, but I forgot to make them public, so we need a PR for that first.

[LucioFranco]

LGTM

[LucioFranco]

Just checking are there any issues exposing a mut reference while this stream is pinned?

[Darksonn]

It's impossible to get an &mut S after having pinned S unless you use unsafe (or if S: Unpin of course), so you can't call this method if you have pinned it.

Though arguably we may want to add an as_pin_mut for non-Unpin wrappers.

[LucioFranco]

good point, this should be fine for now then. I was thinking about the wrong thing, we are not doing any special unpinning here so we can expose the &mut T