io: add `NamedPipe` type on windows

[carllerche]

Mio provides a NamedPipe type. Tokio can provide its own wrapper around it.

The details of the API should be proposed before work on a PR starts. I am not sure if we want to copy mio or tweak the API.

[faern]

Can this be closed thanks to #3760?

[Darksonn]

Yep.