Similarly to the recent commit to do this for networking, there's no reason that
a read on a file descriptor should continue reading until the entire buffer is
full. This makes sense when dealing with literal files, but when dealing with
things like stdin this doesn't make sense.

It is not the case that all programs will always be able to acquire an instance
of the LocalIo borrow, so this commit exposes this limitation by returning
Option<LocalIo> from LocalIo::borrow().

At the same time, a helper method LocalIo::maybe_raise() has been added in order
to encapsulate the functionality of raising on io_error if there is on local I/O
available.

Closes #9126

This adds constructors to pipe streams in the new runtime to take ownership of
file descriptors, and also fixes a few tests relating to the std::run changes
(new errors are raised on io_error and one test is xfail'd).

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.

The general idea is to remove conditions completely from I/O, so in the meantime
remove the read_error condition to mean the same thing as the io_error condition.

This moves as many as I could over to ~Trait instead of ~Typedef. The only
remaining one is the IoFactoryObject which should be coming soon...

This isn't necessary for creating processes (or at least not right now), and
it inherently attempts to expose implementation details.

Who doesn't like a massive renaming?

UnboundedPipeStream is still a newtype since process::set_stdio needs to
look into its internals.

Closes #9667

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).

This reverts commit b8d1fa39, reversing
changes made to f22b4b16.

Conflicts:
	mk/rt.mk
	src/libuv

Closes #6436