Provide direct access to stdin/stderr/stdout from Process

To implement some low level commands we will need direct access to stdin/stdout/stderr.

Need to be able to implement commands like tee.

// tee
main(List<String> args)
one = args[0];
two = args[1];

pOne = one.process;
pTwo = two.process;

pipe(stdin, pOne.stdin);
pipe(stdin, pTwo.stdin);

pOne.start;
pTwo.start;

processUntil(stdin);
}

We should also be able to manage:

'cat somefile'.forEach((line) => '1: line') | 'some other command';

or maybe:

'cat somefile' |. forEach((line) => '1: line') | 'some other command';

We should also be able to simply stream data into a process. In this case we are likely to need to go asyn.

Start by locating the Process implementation and its existing process-streaming or pipe APIs; no file or test is named in the issue. Read how commands are started and how stdin, stdout, and stderr are currently represented. Done should provide direct stream access and support the proposed tee, line-processing, and streaming-input use cases with tests.