refactor!: clean up stdio interface

with an interface no less!

examples/plumbing.php

@@ -11,7 +11,7 @@ $echo = (new Command('echo'))
     ->spawn();
 
 (new Command('cat'))
-    ->stdin(Stdio::stream($echo->stdout))
+    ->stdin($echo->stdout)
     ->status();
 
 // Hello, World!