Implement unix pipes in libnative

There's a few parts to this PR

* Implement unix pipes in libnative for unix platforms (thanks @Geal!)
* Implement named pipes in libnative for windows (terrible, terrible code)
* Remove `#[cfg(unix)]` from `mod unix` in `std::io::net`. This is a terrible name for what it is, but that's the topic of #12093.

The windows implementation was significantly more complicated than I thought it would be, but it seems to be passing all the tests. now.

Closes #11201