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.