Implement process bindings to libuv

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).
auto merge of #9260 : alexcrichton/rust/libuv-processes, r=brson

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).