feat(util): impl listener for either

[saiintbrisson]

Implements the Listener trait for Either. This will ease
adding support for features such as #5678. Allowing users
to accept sockets from any of the two underlying listener
types, like Either<UnixListener, TcpListener>.

Closes #6201.

[Darksonn]

Hmm, I didn't realize that we already have non-trait methods for this.

This makes it more tricky because we shouldn't introduce a breaking change.

[saiintbrisson]

Indeed more tricky, but that was my reasoning behind leaving the original methods. local_addr remains with the same interface, and accept still has an equivalent future and output, unless there's something I'm not seeing.

Probably consider introducing a deprecation notice for local_addr?

[Darksonn]

Actually, this does change the return type of accept from Result<Either<(L::Io, L::Addr), (R::Io, R::Addr)>> to Result<(Either<L::Io, R::Io>, Either<L::Addr, R::Addr>)>.

[Darksonn]

I'm sorry, but I'm going to close this due to the existing methods.

Sorry for not double-checking this before opening the issue.