Access stdin of subprocesses

You can now access the stdin stream of any `cork_subprocess` instance
that you create.  Access is provided via a `cork_stream_consumer`
instance; any data that you write into that stream consumer is sent to
the subprocess's stdin.  We don't do any buffering, or worry in any way
about the buffering of the underlying pipe — you must handle this
yourself if there is any kind of complicated orchestration needed by the
subprocess that you're calling.