Add FileDescriptor.openPipe

[GeorgeLyon]

I had mentally lumped that under process types and calls, but it's not actually blocked on any design work there. We can add this anytime and might make a good starter task.

Originally posted by @milseman in #16 (comment)

One thing that isn't completely clear to me is whether or not it would be useful to introduce a Pipe type (or FileDescriptor.Pipe so as not to cloud the top level namespace). I'm not sure how big of a deal this is, but this could prevent bugs like let (write, read) = try FileDescriptor.openPipe(), as well as allow us to implement close and closeAfter for both file descriptors at once. (Not sure how this affects Windows, @compnerd)

You are correct, this shouldn't be too difficult, at least on the Linux/macOS side of things. I'd be happy to take a pass if we agree on whether or not to include Pipe.