I was seeing a lot of weird behavior with stdin behaving as a tty, and it
doesn't really quite make sense, so instead this moves to using libuv's pipes
instead (which make more sense for stdin specifically).
This prevents piping input to rustc hanging forever.
This involved changing a fair amount of code, rooted in how we access the local
IoFactory instance. I added a helper method to the rtio module to access the
optional local IoFactory. This is different than before in which it was assumed
that a local IoFactory was *always* present. Now, a separate io_error is raised
when an IoFactory is not present, yet I/O is requested.
This is a re-landing of #8645, except that the bindings are *not* being used to
power std::run just yet. Instead, this adds the bindings as standalone bindings
inside the rt::io::process module.
I made one major change from before, having to do with how pipes are
created/bound. It's much clearer now when you can read/write to a pipe, as
there's an explicit difference (different types) between an unbound and a bound
pipe. The process configuration now takes unbound pipes (and consumes ownership
of them), and will return corresponding pipe structures back if spawning is
successful (otherwise everything is destroyed normally).
