Add public split associated function to SimplexStream

[KyleCotton]

Currently the simplex function returns a (ReadHalf<SimplexStream>, WriteHalf<SimplexStream>) which does not conform to the usual convention of (tx, rx) used across the project which can be confusing - see #7175 fixed in #7176.

There are two routes I think we should consider, but I would be very interested in getting some additional input before I submit a PR:

1. Adding another public method potentially SimplexStream::new_split that would return ( WriteHalf<SimplexStream>), ReadHalf<SimplexStream>) to better conform to the convention.

2. Add an extension trait (maybe SplitEx ?) that would have a blanket implementation for T: AsyncRead + AsyncWrite which could enable an API such as tokio::io::simplex(64).split() for simplex and other resources across the project.

Personally I think the first option seems the cleanest and would allow us to then optionally deprecate simplex in a future release and have the public API for SimplexStream feel more familiar.

[maminrayej]

1. Adding another public method potentially SimplexStream::new_split that would return ( WriteHalf), ReadHalf) to better conform to the convention.

I'm afraid having two split functions may add to the confusion instead of mitigating it.

2. Add an extension trait (maybe SplitEx ?) that would have a blanket implementation for T: AsyncRead + AsyncWrite which could enable an API such as tokio::io::simplex(64).split() for simplex and other resources across the project.

What would be the difference between this and tokio::io::split?

[KyleCotton]

I'm afraid having two split functions may add to the confusion instead of mitigating it.

Given the introduction of SimplexStream I would assume that the documentation for the type would now serve as the primary entry point for new users. Potentially they would see the new_* methods first?

Agreed that having two splitting function could be confusing. I would suggest deprecating the simplex function (in some future release) in favour of the new SimplexStream API and make it more familiar to the other reader/writer APIs i.e. (tx, rx).

What would be the difference between this and tokio::io::split?

No functional difference. Would just enable method chaining uses.

I guess more generally in an ideal world we'd just swap the reader/writer in the simplex function. In reality this would be a major breaking change. I'd be eager to hear other suggestions to improve the ergonomics without having the major version bump.